drm/msm/sde: add reg bus vote for histogram and AD

This change will add AHB vote for histogram and AD. This is
needed to access the histogram and AD registers in respective
histogram and AD read handler.

Change-Id: I2e99f8d34722cb0cf9c9516798b20641c1e1cb5a
Signed-off-by: Ch Ganesh Kumar <chganesh@codeaurora.org>
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>