sde-drm: Protect access to prop_val_map to avoid crash

DRMCrtc's map (prop_val_map) can be accessed by multiple
threads simultaneously which can cause deadlocks or segfault
errors. This changes adds a mutex lock to protect access to
the object to avoid such scenarios.

Change-Id: I6977a5f2e917e883919a307938032fd05ef93133
1 file changed