msm_fb: Fix blending logic for formats with per-pixel alpha

Currently if a layer with per-pixel alpha (eg. RGBA) is fed
for composition with z-order as zero (FG), per-pixel alpha
blending is applied along with the BG frame present on frame
buffer and hence we see the previous frame on the panel. Hence,
apply constant alpha blending if these formats are fed as FG.

(cherry picked from commit dd10bf1007a50b8b6956a5470ff92a2b4ddf9971)

Change-Id: I531410aa573ab2e209652a2b2fbebe5a6b70b51a
CRs-fixed: 405802
CRs-fixed: 435800
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>
Merge "msm_fb: Fix blending logic for formats with per-pixel alpha"