drm/imx: hdmi: fix HDMI setup to allow modes larger than FullHD

This worked before the dw-hdmi bridge code was changed to validate
the setup data more strictly. Add back support for modes with a
pixel clock up to 216MHz. Even higher clocks should work, but we
are missing the required setup data for now.

Also change the mode validate callbacks to disallow modes with
higher pixelclocks, so we don't end up failing the modeset later
on.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>