staging: drm/imx: ipuv3-crtc: immediately update crtc->fb in ipu_page_flip

Since commit 8cf1e981
("drm: Add consistency check for page-flipping") drm_mode_page_flip_ioctl
contains a WARN_ON that triggers if the .page_flip callback didn't update
the crtc->fb pointer to the new framebuffer immediately.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>