imx-drm: ipuv3-crtc: Implement mode_fixup

Ask the IPU display interface, via ipu_di_adjust_videomode(), to
adjust a video mode to meet any DI restrictions. The function takes
a subsystem independent videomode, so the drm_display_mode must be
converted to videomode first, and then the adjusted mode converted
back to a drm_display_mode.

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>