drm/msm: Clear the mode private flags before setting

Clear the display mode private flags before assigning
the new value. These flags retain the values from the
previous mode set which could be stale in cases where
the mode is different across the hotplugs.

Change-Id: I4bd7021970737e5ae22bade3074d8debfeddc7b3
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>