drm/msm/sde: avoid alpha fast clear on z_stage 0

Avoid alpha fast clear enable on z_stage 0 because
some client may stage non-opaque layer on this stage
with fast clear UBWC data on RGBA plane.

Change-Id: Ida5747401c5855875ed32c4b75163cd8b872dbbe
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>