drm: rcar-du: Consider plane to CRTC associations in the plane allocator

Hardware planes are driven by the timing generator of the CRTC they are
associated to. Changing the association requires restarting the CRTC
group that the plane belongs to, resulting in flicker on the other CRTC.

To avoid flicker as much as possible, try to allocate planes first from
the free planes already associated with the target CRTC. If allocation
fails then fall back to allocation from all free planes.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>