msm: mdss: keep mdp clk on during overlay kick off

This will reduce the delay caused by turning on/off mdp clk multiple
times during kickoff. Currently during kickoff, mdp pipe registers and
flush registers are programmed each with mdp clk on/off. We only need
turn mdp clk on/off once to program those registers. Increase scope of
mdp clock vote to keep them on during the entire programming sequence.

Change-Id: I5270c5e7aeef2f16c6a64d56827a73a8cb380fc5
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>