drm: rp1: vec: Support more video modes in the RP1 VEC driver

Support a wider range of pixel clock rates. The driver will round
pixclock up to 108MHz/n but tries to honour the desired image width
and position (of the centre of the display relative to HSYNC_STARTs).
This adds complexity but removes the need for separate 13.5MHz and
15.428MHz modes.

Support "fake" double-rate progressive modes (in which only every
2nd scanline is displayed). To work around aspect ratio issues.

Add Monochrome TV mode support. Add "vintage" modes (544x380i for
System A; 848x738i for System E) when configured for Monochrome.

Add a way to create a "custom" display mode from a module parameter.

Signed-off-by: Nick Hollinghurst <nick.hollinghurst@raspberrypi.com>