drm/amd/display: half bandwidth for YCbCr420 during validation

[Why]
used to be unable to run 4:2:0 if using a dongle because 4k60 bandwidth
exceeded dongle caps

[How]
half pixel clock during comparison to dongle cap. *Could get stuck on black
screen on monitor that don't support 420 but will be selecting 420 as
preferred mode*

Signed-off-by: Martin Leung <martin.leung@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Aidan Wood <Aidan.Wood@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>