drm/fsl-dcu: enable IRQ before drm_atomic_helper_resume()

The resume helpers wait for a vblank to occurre hence IRQ need
to be enabled. This avoids a warning as follows during resume:
  WARNING: CPU: 0 PID: 314 at drivers/gpu/drm/drm_atomic_helper.c:1249 drm_atomic_helper_wait_for_vblanks.part.1+0x284/0x288
  [CRTC:28:crtc-0] vblank wait timed out

Signed-off-by: Stefan Agner <stefan@agner.ch>