drm: rcar-du: Arm the page flip event after queuing the page flip

The page flip event is armed in the atomic begin handler, creating a
race condition with the frame end interrupt that could send the event
before the atomic operation actually completes. To avoid that, arm the
event in the atomic flush handler after queuing the page flip.

This change doesn't fully close the race window, as the frame end
interrupt could be generated before the page flip is committed to
hardware but only handled after the event is armed. However, the race
window is now much smaller.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>