drm/vc4: Enable background color fill when necessary

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