drm/imx: dw_hdmi-imx: add mode_valid callback prune unsupported modes

This patch limits the pixel clock to 13.4 MHz - 266 MHz for i.MX6Q
and 13.5 MHz - 270 MHz for i.MX6DL, which is the range documented
in the HDMI Transmitter chapter of the respective reference manuals.

Without this patch, when connected to a monitor capable of 2160p60
modes, dw_hdmi will happily report this mode and the IPU code will
cause a division by zero in ipu_di_config_clock when trying to figure
out how to divide the 264 MHz HSP clock down to ~600 MHz.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>