drm/vc4: Check if plane requires background fill

Considering a single plane only, we have to enable background color
when the plane has an alpha format and could be blending from the
background or when it doesn't cover the entire screen.

Changes from v1:
 - Drop unrelated change
 - Move needs_bg_fill to 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/1520556817-97297-3-git-send-email-stschake@gmail.com