drm/msm/dsi: Implement RPM suspend/resume callbacks

The bus clocks are always enabled/disabled along with the power
domain, so move it to the runtime suspend/resume ops. This cleans
up the clock code a bit. Get rid of the clk_mutex mutex since it
isn't needed.

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