msm_fb: Make sure MDP clock is ON during register access

Change to make sure that MDP clock is ON while accessing
MDP fixed arbitration read and write selection
registers. This avoids target freeze due to unclocked
register access.

CRs-fixed: 505154
Change-Id: I1d2e654c7b97ca51e42611a5bd3aefd65421bcf4
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>