drm: renesas: shmobile: Wait for page flip when turning CRTC off

Turning a CRTC off will prevent a queued page flip from ever completing,
potentially confusing userspace.  Wait for queued page flips to complete
before turning the CRTC off to avoid this.

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/c97d5859c43fa36043c61de28d67688ebe345092.1694767209.git.geert+renesas@glider.be