drm/msm: clear deep color mode flags when updating modes

Currently, deep color mode flags are not being cleared
across connect-disconnect of HDMI cable. This leads to
stale information being present on the next successful
connection.

Clear the stale deep color mode flags before updating the
modes during a connection event.

Change-Id: I90d494a67dfb1c3fd464f1e1ffb24592bd712702
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>