mdp4_overlay: Don't always call ion_unmap_iommu

Don't call ion_unmap_iommu if DISPLAY_READ_DOMAIN and GEN_POOL equal 2:

<4>[ 3143.147399] ------------[ cut here ]------------
<4>[ 3143.147583] WARNING: at drivers/gpu/ion/ion.c:750 ion_unmap_iommu+0xb8/0xe0()
<4>[ 3143.147674] ion_unmap_iommu: (2,2) was never mapped for db20c060
<4>[ 3143.147827] Modules linked in: dhd vibrator
<4>[ 3143.148132] [<c004e5ac>] (unwind_backtrace+0x0/0xf8) from [<c00bc9d4>] (warn_slowpath_common+0x4c/0x64)
<4>[ 3143.148315] [<c00bc9d4>] (warn_slowpath_common+0x4c/0x64) from [<c00bca80>] (warn_slowpath_fmt+0x30/0x40)
<4>[ 3143.148468] [<c00bca80>] (warn_slowpath_fmt+0x30/0x40) from [<c02b521c>] (ion_unmap_iommu+0xb8/0xe0)
<4>[ 3143.148651] [<c02b521c>] (ion_unmap_iommu+0xb8/0xe0) from [<c0258330>] (mdp4_overlay_iommu_unmap_freelist+0x7c/0xd4)
<4>[ 3143.148834] [<c0258330>] (mdp4_overlay_iommu_unmap_freelist+0x7c/0xd4) from [<c025d804>] (mdp4_lcdc_pipe_commit+0x188/0x370)
<4>[ 3143.149017] [<c025d804>] (mdp4_lcdc_pipe_commit+0x188/0x370) from [<c025cf90>] (mdp4_overlay_commit+0x90/0xac)
<4>[ 3143.149200] [<c025cf90>] (mdp4_overlay_commit+0x90/0xac) from [<c024f17c>] (msm_fb_commit_thread+0x84/0x420)
<4>[ 3143.149383] [<c024f17c>] (msm_fb_commit_thread+0x84/0x420) from [<c00d6408>] (kthread+0x80/0x88)
<4>[ 3143.149536] [<c00d6408>] (kthread+0x80/0x88) from [<c0049960>] (kernel_thread_exit+0x0/0x8)
<4>[ 3143.149688] ---[ end trace 0cd62fb71d81fcdb ]---

Signed-off-by: Sultanxda <sultanxda@gmail.com>