video: mdss: remove erroneous mutex_unlock

Noticed the following warning:
WARNING: at ../../../../../../kernel/oneplus/msm8974/drivers/video/msm/mdss/mdss_livedisplay.c:145 mdss_livedisplay_worker+0x494/0x580()
[<c0110b0c>] (unwind_backtrace+0x0/0x160) from [<c01c0f8c>] (warn_slowpath_null+0x40/0x64)
[<c01c0f8c>] (warn_slowpath_null+0x40/0x64) from [<c04d03f0>] (mdss_livedisplay_worker+0x494/0x580)
[<c04d03f0>] (mdss_livedisplay_worker+0x494/0x580) from [<c01df1b8>] (process_one_work+0x174/0x4f4)
[<c01df1b8>] (process_one_work+0x174/0x4f4) from [<c01dfd7c>] (worker_thread+0x1ac/0x44c)
[<c01dfd7c>] (worker_thread+0x1ac/0x44c) from [<c01e6598>] (kthread+0xc8/0xd4)
[<c01e6598>] (kthread+0xc8/0xd4) from [<c010a824>] (kernel_thread_exit+0x0/0x8)

OPO-713

Change-Id: I0f0ca1bdfd27f2196b4ab9d978653a39515547d3