fix warning caused by is_active(..) on a ring buffer bo

We use the gpu variable of the bo to keep track of the gpu if the gem
is used as internal ring buffer. In this case it is wrong to check
if the bo is active or not.

[ 1986.992014] ------------[ cut here ]------------
[ 1986.996711] WARNING: CPU: 2 PID: 2463 at drivers/staging/etnaviv/etnaviv_gem.c:565 etnaviv_gem_free_object+0x190/0x198 [etnaviv]()
[ 1987.011058] Modules linked in: etnaviv(C-) [last unloaded: etnaviv]
[ 1987.017409] CPU: 2 PID: 2463 Comm: rmmod Tainted: G        WC     3.18.0-rc5+ #8
[ 1987.024938] [<c0013d6c>] (unwind_backtrace) from [<c001133c>] (show_stack+0x10/0x14)
[ 1987.032795] [<c001133c>] (show_stack) from [<c04f7b80>] (dump_stack+0x74/0x90)
[ 1987.040095] [<c04f7b80>] (dump_stack) from [<c0024c2c>] (warn_slowpath_common+0x64/0x88)
[ 1987.048226] [<c0024c2c>] (warn_slowpath_common) from [<c0024c6c>] (warn_slowpath_null+0x1c/0x24)
[ 1987.057113] [<c0024c6c>] (warn_slowpath_null) from [<bf011a9c>] (etnaviv_gem_free_object+0x190/0x198 [etnaviv])
[ 1987.067314] [<bf011a9c>] (etnaviv_gem_free_object [etnaviv]) from [<c02d6af4>] (drm_gem_object_free+0x28/0x30)
[ 1987.077418] [<c02d6af4>] (drm_gem_object_free) from [<bf01293c>] (etnaviv_gpu_unbind+0xd0/0x11c [etnaviv])
[ 1987.087196] [<bf01293c>] (etnaviv_gpu_unbind [etnaviv]) from [<c02fa870>] (component_unbind+0x30/0x68)
[ 1987.096575] [<c02fa870>] (component_unbind) from [<c02fa918>] (component_unbind_all+0x70/0xa4)
[ 1987.105273] [<c02fa918>] (component_unbind_all) from [<bf010678>] (etnaviv_unload+0x60/0x74 [etnaviv])
[ 1987.114673] [<bf010678>] (etnaviv_unload [etnaviv]) from [<c02da814>] (drm_dev_unregister+0x24/0x98)
[ 1987.123905] [<c02da814>] (drm_dev_unregister) from [<c02db1dc>] (drm_put_dev+0x28/0x64)
[ 1987.131983] [<c02db1dc>] (drm_put_dev) from [<c02fad34>] (take_down_master+0x24/0x44)
[ 1987.139884] [<c02fad34>] (take_down_master) from [<c02fadc4>] (component_del+0x70/0xa8)
[ 1987.147941] [<c02fadc4>] (component_del) from [<bf012864>] (etnaviv_gpu_platform_remove+0x14/0x1c [etnaviv])
[ 1987.157867] [<bf012864>] (etnaviv_gpu_platform_remove [etnaviv]) from [<c02ffbe0>] (platform_drv_remove+0x14/0x18)
[ 1987.168317] [<c02ffbe0>] (platform_drv_remove) from [<c02fe824>] (__device_release_driver+0x58/0xac)
[ 1987.177528] [<c02fe824>] (__device_release_driver) from [<c02fefe4>] (driver_detach+0xac/0xb0)
[ 1987.186210] [<c02fefe4>] (driver_detach) from [<c02fe6dc>] (bus_remove_driver+0x4c/0x90)
[ 1987.194396] [<c02fe6dc>] (bus_remove_driver) from [<bf015500>] (etnaviv_exit+0xc/0x20 [etnaviv])
[ 1987.203306] [<bf015500>] (etnaviv_exit [etnaviv]) from [<c0082748>] (SyS_delete_module+0x120/0x1b4)
[ 1987.212433] [<c0082748>] (SyS_delete_module) from [<c000e660>] (ret_fast_syscall+0x0/0x30)
[ 1987.220769] ---[ end trace c70233db5ee296dd ]---
[ 1987.225550] ------------[ cut here ]------------
[ 1987.230305] WARNING: CPU: 2 PID: 2463 at drivers/staging/etnaviv/etnaviv_gem.c:565 etnaviv_gem_free_object+0x190/0x198 [etnaviv]()
[ 1987.242108] Modules linked in: etnaviv(C-) [last unloaded: etnaviv]
[ 1987.248457] CPU: 2 PID: 2463 Comm: rmmod Tainted: G        WC     3.18.0-rc5+ #8
[ 1987.255938] [<c0013d6c>] (unwind_backtrace) from [<c001133c>] (show_stack+0x10/0x14)
[ 1987.263768] [<c001133c>] (show_stack) from [<c04f7b80>] (dump_stack+0x74/0x90)
[ 1987.271067] [<c04f7b80>] (dump_stack) from [<c0024c2c>] (warn_slowpath_common+0x64/0x88)
[ 1987.279195] [<c0024c2c>] (warn_slowpath_common) from [<c0024c6c>] (warn_slowpath_null+0x1c/0x24)
[ 1987.288077] [<c0024c6c>] (warn_slowpath_null) from [<bf011a9c>] (etnaviv_gem_free_object+0x190/0x198 [etnaviv])
[ 1987.298288] [<bf011a9c>] (etnaviv_gem_free_object [etnaviv]) from [<c02d6af4>] (drm_gem_object_free+0x28/0x30)
[ 1987.308389] [<c02d6af4>] (drm_gem_object_free) from [<bf01293c>] (etnaviv_gpu_unbind+0xd0/0x11c [etnaviv])
[ 1987.318141] [<bf01293c>] (etnaviv_gpu_unbind [etnaviv]) from [<c02fa870>] (component_unbind+0x30/0x68)
[ 1987.327497] [<c02fa870>] (component_unbind) from [<c02fa918>] (component_unbind_all+0x70/0xa4)
[ 1987.336161] [<c02fa918>] (component_unbind_all) from [<bf010678>] (etnaviv_unload+0x60/0x74 [etnaviv])
[ 1987.345513] [<bf010678>] (etnaviv_unload [etnaviv]) from [<c02da814>] (drm_dev_unregister+0x24/0x98)
[ 1987.354687] [<c02da814>] (drm_dev_unregister) from [<c02db1dc>] (drm_put_dev+0x28/0x64)
[ 1987.362725] [<c02db1dc>] (drm_put_dev) from [<c02fad34>] (take_down_master+0x24/0x44)
[ 1987.370587] [<c02fad34>] (take_down_master) from [<c02fadc4>] (component_del+0x70/0xa8)
[ 1987.378615] [<c02fadc4>] (component_del) from [<bf012864>] (etnaviv_gpu_platform_remove+0x14/0x1c [etnaviv])
[ 1987.388498] [<bf012864>] (etnaviv_gpu_platform_remove [etnaviv]) from [<c02ffbe0>] (platform_drv_remove+0x14/0x18)
[ 1987.398888] [<c02ffbe0>] (platform_drv_remove) from [<c02fe824>] (__device_release_driver+0x58/0xac)
[ 1987.408056] [<c02fe824>] (__device_release_driver) from [<c02fefe4>] (driver_detach+0xac/0xb0)
[ 1987.416712] [<c02fefe4>] (driver_detach) from [<c02fe6dc>] (bus_remove_driver+0x4c/0x90)
[ 1987.424846] [<c02fe6dc>] (bus_remove_driver) from [<bf015500>] (etnaviv_exit+0xc/0x20 [etnaviv])
[ 1987.433679] [<bf015500>] (etnaviv_exit [etnaviv]) from [<c0082748>] (SyS_delete_module+0x120/0x1b4)
[ 1987.442763] [<c0082748>] (SyS_delete_module) from [<c000e660>] (ret_fast_syscall+0x0/0x30)
[ 1987.451052] ---[ end trace c70233db5ee296de ]---

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>