msm: display: Reset BG_alpha register while alpha blending

Reset BG_alpha register when pre-multiplied alpha is not
set for a layer during blit request.

CRs-fixed: 351158
Change-Id: If103af384d59c3349cb2ef556a497c709d9280a2
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>