drm/msm/hdmi: HDMI 8996 PHY/PLL support

Add support for the HDMI PHY/PLL found in MSM8996/APQ8096.

Unlike the previous PHYs supported in the driver, this doesn't need
the powerup/powerdown ops. The PLL prepare/unprepare clock ops
enable/disable the phy itself.

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