staging: drm/imx: fix pageflip events during device close

During a device close the drm core frees all pending events in
drm_events_release(). If at that time a pageflip is pending the
interrupt handler will try to complete the now unitialized
event resulting in a NULL pointer exception. Seen on imx-drm
when userspace is killed during a page flip.

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