drm/vc4: Move CRTC state to header

We need to access the channel for configuring our CTM hardware.

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/1523479755-20812-4-git-send-email-stschake@gmail.com