drm/meson: Add YUV420 output support

This patch adds support for the YUV420 output from the Amlogic Meson SoCs
Video Processing Unit to the HDMI Controller.

The YUV420 is obtained by generating a YUV444 pixel stream like
the classic HDMI display modes, but then the Video Encoder output
can be configured to down-sample the YUV444 pixel stream to a YUV420
stream.
In addition if pixel stream down-sampling, the Y Cb Cr components must
also be mapped differently to align with the HDMI2.0 specifications.

This mode needs a different clock generation scheme since the TMDS PHY
clock must match the 10x ratio with the YUV420 pixel clock, but
the video encoder must run at 2x the pixel clock.

This patch enables the bridge bus format negociation, and handles
the YUV420 case if selected by the negociation.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Jernej Škrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200304104052.17196-12-narmstrong@baylibre.com