drm/msm/dpu: use feature bit for LM combined alpha check

Rather than checking hwversion, follow the usual patter and add special
bit to the lm->features to check whether the LM has combined or separate
alpha registers. While we are at it, rename
dpu_hw_lm_setup_blend_config_sdm845() to
dpu_hw_lm_setup_blend_config_combined_alpha().

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/488155/
Link: https://lore.kernel.org/r/20220602202447.1755115-2-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>