drm/imx: use drm_send_vblank_event() helper

Also, slightly changes the behavior to always put the vblank irq,
even if userspace did not request a vblank event.  As far as I
can tell, the previous code would leak a vblank irq refcnt if
userspace requested a pageflip without event.

Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>