msm: display: Add DSI clock control for MDP GDHS power collapse

Enable DSI clock and regulator before enabling/disabling power collapse
for MDP GDHS.

Change-Id: I9effb5b4e9ca8033c51085260c8fb9f32c422dd3
Signed-off-by: Ravishangar Kalyanam <rkalya@codeaurora.org>
Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>