imx-drm: lock scanout transfers for consecutive bursts

- Lock the IDMAC scanout channel for multiple back-to-back bursts if possible,
  to improve memory bandwidth utilisation.
- Replace a few occurences of state->fb with the already existing local fb
  variable in ipu_plane_atomic_update

* tag 'imx-drm-next-2017-07-18' of git://git.pengutronix.de/git/pza/linux:
  drm/imx: lock scanout transfers for consecutive bursts
  drm/imx: ipuv3-plane: use fb local variable instead of state->fb