drm/msm/sde: modify stage for dim layer during atomic set

Stage property is modified for dimlayer during atomic
set property phase. This avoids any commit failure occurred due
to dimlayer checks during atomic check phase.

Change-Id: I4ff3d83a5aa9d6446fd4955f6c29854acf93bc68
Signed-off-by: Prashant Singh <prasin@codeaurora.org>