drm/msm/dsi-staging: Expose panel bit depth

Add changes to expose panel bit depth to HAL.
This is done to ensure other sub-systems send
content as per panel format such that there is
no conversion needed.

Change-Id: I1104445136928e1af940b0f68ac426f0f2949f92
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>