sdm: Don't block draw-cycle thread during SetDisplayBrightness

Driver makes sure that Panel commands are serialized to any frame
update on Panel. SetDisplayBrightness API doesn't need to acquire
locker_ lock which blocks draw-cycle thread and causes frame drops.
Define brightness_lock_ to serialize Set and Get Panel brightness.

CRs-Fixed: 2446089
Change-Id: I963f1b3ddcd4765ddff805a373f70cdd550fa993
3 files changed