LF-4506-1: drm: bridge: cdns-hdmi: support YUV420 only video mode

commit 8a22abda3c56af9189fa3bdcddd6d0f9677b014d from
https://github.com/nxp-imx/linux-imx

Set flag ycbcr_420_allowed to support YUV420 only video mode.

Signed-off-by: Sandor Yu <Sandor.yu@nxp.com>
Reviewed-by: Laurentiu Palcu <laurentiu.palcu@oss.nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>