drm/msm/hdmi: Add basic HDMI support for msm8996

The HDMI controller is new in MDP5 v1.7. As of now, this change
doesn't reflect the novelty and only adds the basics so the probe
gets triggered.

Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>