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.

CRs-fixed: 405802
CRs-fixed: 435800
Change-Id: I4e00743e7f13189b1830d4d0e655f186ffa590cd
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>