drm/msm/sde: Expose pgc driver interface

MSM sde module supports gamma correction block. Driver exposes the
interface to user-space clients to program the block.

Change-Id: Ib12947d9aa4435f42d72a704fbac97e74b66353b
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>