drm/msm: clear deep color bits in the HDMI controller

Clear the deep color bits in the HDMI controller in case
the mode does not need OR support deep color.

Otherwise it causes issues while switching across modes where
the sink supports deep color in one of the modes but not in the
other.

Change-Id: I8accf00e89868ccaa781a3f18eb9a662d9c9235c
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>