disp: msm: sde: use helper to determine plane pre-downscale cap

The pre-downscale capability is checked in multiple places
within sde_plane.c file. Add a helper function to check this
capability flag instead of manually checking this bit.

Change-Id: I21f818a9d81dd63e5eb3da248532904cfa55c838
Signed-off-by: Steve Cohen <cohens@codeaurora.org>
Signed-off-by: Jayaprakash <jmadiset@codeaurora.org>