drm/msm/dsi: Set up runtime PM for DSI

Call the pm_runtime_get/put API where we need the clocks enabled.

The main entry/exit points are 1) enabling/disabling the DSI bridge
and 2) Sending commands from the DSI host to the device.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>