drm: fix a use-after-free when GPU acceleration disabled

commit b7ea85a4fed37835eec78a7be3039c8dc22b8178 upstream.

When GPU acceleration is disabled, drm_vblank_cleanup() will free the
vblank-related data, such as vblank_refcount, vblank_inmodeset, etc.
But we found that drm_vblank_post_modeset() may be called after the
cleanup, which use vblank_refcount and vblank_inmodeset. And this will
cause a kernel panic.

Fix this by return immediately if dev->num_crtcs is zero. This is the
same thing that drm_vblank_pre_modeset() does.

Call trace of a drm_vblank_post_modeset() after drm_vblank_cleanup():
[   62.628906] [<ffffffff804868d0>] drm_vblank_post_modeset+0x34/0xb4
[   62.628906] [<ffffffff804c7008>] atombios_crtc_dpms+0xb4/0x174
[   62.628906] [<ffffffff804c70e0>] atombios_crtc_commit+0x18/0x38
[   62.628906] [<ffffffff8047f038>] drm_crtc_helper_set_mode+0x304/0x3cc
[   62.628906] [<ffffffff8047f92c>] drm_crtc_helper_set_config+0x6d8/0x988
[   62.628906] [<ffffffff8047dd40>] drm_fb_helper_set_par+0x94/0x104
[   62.628906] [<ffffffff80439d14>] fbcon_init+0x424/0x57c
[   62.628906] [<ffffffff8046a638>] visual_init+0xb8/0x118
[   62.628906] [<ffffffff8046b9f8>] take_over_console+0x238/0x384
[   62.628906] [<ffffffff80436df8>] fbcon_takeover+0x7c/0xdc
[   62.628906] [<ffffffff8024fa20>] notifier_call_chain+0x44/0x94
[   62.628906] [<ffffffff8024fcbc>] __blocking_notifier_call_chain+0x48/0x68
[   62.628906] [<ffffffff8042d990>] register_framebuffer+0x228/0x260
[   62.628906] [<ffffffff8047e010>] drm_fb_helper_single_fb_probe+0x260/0x314
[   62.628906] [<ffffffff8047e2c4>] drm_fb_helper_initial_config+0x200/0x234
[   62.628906] [<ffffffff804e5560>] radeon_fbdev_init+0xd4/0xf4
[   62.628906] [<ffffffff804e0e08>] radeon_modeset_init+0x9bc/0xa18
[   62.628906] [<ffffffff804bfc14>] radeon_driver_load_kms+0xdc/0x12c
[   62.628906] [<ffffffff8048b548>] drm_get_pci_dev+0x148/0x238
[   62.628906] [<ffffffff80423564>] local_pci_probe+0x5c/0xd0
[   62.628906] [<ffffffff80241ac4>] work_for_cpu_fn+0x1c/0x30
[   62.628906] [<ffffffff802427c8>] process_one_work+0x274/0x3bc
[   62.628906] [<ffffffff80242934>] process_scheduled_works+0x24/0x44
[   62.628906] [<ffffffff8024515c>] worker_thread+0x31c/0x3f4
[   62.628906] [<ffffffff802497a8>] kthread+0x88/0x90
[   62.628906] [<ffffffff80206794>] kernel_thread_helper+0x10/0x18

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Binbin Zhou <zhoubb@lemote.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Acked-by: Paul Menzel <paulepanter@users.sourceforge.net>
Signed-off-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>