drm: rcar-du: Wait for flip completion instead of vblank in commit tail

drm: rcar-du: Wait for flip completion instead of vblank in commit tail

Page flips can take more than one vertical blanking to complete if
arming the page flips races with the vertical blanking interrupt.
Waiting for one vblank to complete the atomic commit in the commit tail
handler is thus incorrect, and can lead to framebuffers being released
while still being scanned out.

Fix this by waiting for flip completion instead, using the
drm_atomic_helper_wait_for_flip_done() helper.

Fixes: 0d230422d256 ("drm: rcar-du: Register a completion callback with VSP1")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>