msm: mdss: disable HDMI block before HDMI core off

Switching HDMI display mode many times, could find
sometimes color distortion or no signal display.
If disable HDMI block before setting HDMI core off,
that could be fixed.

Change-Id: Ib0e8002f59e017a81298d4a5a39736a7516e3435
Signed-off-by: Yuan Zhao <yzhao@codeaurora.org>