drm/msm/dpu: simplify bg_alpha selection

drm/msm/dpu: simplify bg_alpha selection

In order to be more obvious in fg_alpha / bg_alpha handling during the
blending programming drop the default setting for background alpha value
and set it explicitly in all cases.

Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/697896/
Link: https://lore.kernel.org/r/20260112-dpu-rework-alpha-v2-1-d168785911d5@oss.qualcomm.com