msm_fb: display: add mdp clock control to histogram enable/disable

Add mdp clock control to histogram enable and disable block to
prevent system from crashing when accessing register with mdp
clocks off.

Change-Id: Ide808b332b5456f3fc639fcbb0d03d5cad0e387f
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>