drm/msm/sde: use const alpha for wb if pipe alpha not supported

Writeback can source alpha from upper pipe or from constant
alpha. When pipe alpha is not supported, use constant alpha
to supply alpha value for writeback.

Change-Id: I4381aeb35bc1ef4c18e70174880b223a61e89024
Signed-off-by: Alan Kwong <akwong@codeaurora.org>