drm/imx: format modifier support

- Add tiled prefetch support to PRE
- Add format modifier support to PRG and imx-drm-core
- Use runtime PM to control PRG clock
- Allow building ipu-v3 under COMPILE_TEST

* tag 'imx-drm-next-2018-01-02' of git://git.pengutronix.de/git/pza/linux:
  gpu: ipu-v3: allow to build with COMPILE_TEST
  drm/imx: advertise supported plane format modifiers
  drm/imx: add FB modifier support
  gpu: ipu-v3: prg: add modifier support
  gpu: ipu-v3: pre: add tiled prefetch support
  gpu: ipu-v3: prg: switch to runtime PM