Using the hint from the plane state, we turn on the background color
to avoid display corruption from planes blending with the background.

Changes from v1:
 - Use needs_bg_fill from plane state
Signed-off-by: Stefan Schake <stschake@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1520556817-97297-5-git-send-email-stschake@gmail.com