drm/vc4: Make the CRTCs cooperate on allocating display lists.

So far, we've only ever lit up one CRTC, so this has been fine.  To
extend to more displays or more planes, we need to make sure we don't
run our display lists into each other.

Signed-off-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit d8dbf44f)