From: Hsiao Chien Sung <shawn.sung@xxxxxxxxxxxx>
Support "Pre-multiplied" alpha blending mode in Mixer.
Before this patch, only the coverage mode is supported.
To replace the default setting that is set in mtk_ethdr_config(),
we change mtk_ddp_write_mask() to mtk_ddp_write(), and this change will
also reset the NON_PREMULTI_SOURCE bit that was assigned in
mtk_ethdr_config(). Therefore, we must still set NON_PREMULTI_SOURCE bit
if the blend mode is not DRM_MODE_BLEND_PREMULTI.
Signed-off-by: Hsiao Chien Sung <shawn.sung@xxxxxxxxxxxx>