drm/sun4i: Add support for plane alpha

Our backend supports a per-plane alpha property. Support it through our new
helper.

Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/13e89f0d2f2b55752a22eb8c4f37f325246a3a9c.1523432341.git-series.maxime.ripard@bootlin.com