drm/msm/sde: Add pgc programming support through ahb

SDE block can support pgc using via reg-dma or ahb interface. If sde
block doesn't have reg-dma block, install the ahb programming function
pointer during the driver probe. Change adds support for pgc programming
via ahb interface.

Change-Id: I6c6299e2e58afc1add263cd98aeb4cdee8a87a62
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>