drm: bridge/dw_hdmi: add support for interlaced video modes

Add support for interlaced video modes to the dw_hdmi bridge.  This
mainly involves halving the vertical parameters to be programmed into
the bridge registers, and setting the interlace_allowed connector flag.

This brings working 1080i support.  However, 480i and 576i fail to
work due to the lack of proper pixel repetition support, which is not
trivial to add due to the tabular PLL parameterisation.  Hence, we
filter out these modes in our mode_valid() method.

Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>