drm/msm/sde: check both the rect's of sDMA before enabling UBWC const alpha

Both the rectangles of a sDMA 2.0 HW pipe share the same UBWC static
control register where constant alpha bit will be set. This change
checks both rectangle eligibility before setting the bit.

Change-Id: I48e504387c4ebc0b6bdc06205f8ac3b7c0294cb7
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>