drm/msm/dpu: get rid of DPU_MIXER_COMBINED_ALPHA

drm/msm/dpu: get rid of DPU_MIXER_COMBINED_ALPHA

Continue migration to the MDSS-revision based checks and replace
DPU_MIXER_COMBINED_ALPHA feature bit with the core_major_ver >= 4 check.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/655398/
Link: https://lore.kernel.org/r/20250522-dpu-drop-features-v5-21-3b2085a07884@oss.qualcomm.com