drm/msm/hdmi: Set up runtime PM for HDMI

Enable rudimentary runtime PM in the HDMI driver. We can't really do
agressive PM toggling at the moment because we need to leave the hpd
clocks enabled all the time. There isn't much benefit of creating
suspend/resume ops to toggle clocks either.

We just make sure that we configure the power domain in the HDMI bridge's
enable/disable paths, and the HDMI connector's detect() op.

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