msm: mdss: fix un-clocked register access when rotator is used

When rotator is used, write-back interface is used to write data back to
memory. When wait function for write-back is returned, it will remove its
vote for mdp clocks. So during command mode panels, it is possible that
rotator is used without panel itself being turned on. In this scenario
register access after wait function leads to un-clocked access and L2
kernel panic. Fix this by voting for mdp clks before any register
accesses following wait function.

Change-Id: I9b248d39132f197fc8abd23ab3a23a9eb105ba22
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>