drm/msm/dpu: Ignore alpha for XBGR8888 format

Alpha enable in the pixel format will help in
selecting the blend rule. By keeping alpha enable
to true we are allowing foreground alpha to blend
with the layer. If alpha is don't care, then we
should not allow pixel alpha to be part of blend
equation.

Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>