drm/vc4: Move plane state to header

We need to reference it from the CRTC to make a decision for enabling
background color fill.

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/1520556817-97297-4-git-send-email-stschake@gmail.com