drm/msm/sde: fix post_commit assignment in secure transitions

The transition from secure to non-secure is handled during
post commit for video mode panels. This is done based on the
post_commit flag set during the transition checks. Fix the
wrong bit operation used in setting the post_commit flag to
avoid unexpected behavior, during the transition.

Change-Id: I949be0b689a97f96b60bfb01863298071281e24c
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>