drm: omapdrm: Cancel pending page flips when closing device

Pending page flips must be cancelled when closing the device, otherwise
their completion at next vblank will result in nasty effects, including
possible oopses due to resources required to complete the page flip
being freed.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>