drm/bridge: dw-hdmi: change YUV420 selection logic at clock setup

Right now clocking value selection code is prioritising RGB, YUV444 modes
over YUV420 for HDMI2 sinks. However, because of the bus format selection
procedure in dw-hdmi, for HDMI2 sinks YUV420 is the format that will always
be picked during the drm bridge chain check stage.

Later on dw_hdmi_setup will configure a colour space based on the bus
format that doesn't match the pixel value we had calculated as described
above.

Fix it by bringing back dw-hdmi bus format check when picking the right
pixel clock.

Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Acked-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/6230bfae2cd97cf6527fc62ba5c850464919ccf8.1687702042.git.adrian.larumbe@collabora.com