[PATCH 2/3] Restructure output format computation for better expandability

[Werner Sembach]

Couples the decission between RGB and YCbCr420 mode and the check if the port
clock can archive the required frequency. Other checks and configuration steps
that where previously done in between can also be done before or after.

This allows for are cleaner implementation of retrying different color
encodings.

Slight change in behaviour: If YCbCr420 is not allowed but display is YCbCr420
only it no longer fails, but just prints an error and tries to fallback on RGB.

Signed-off-by: Werner Sembach <wse@xxxxxxxxxxxxxxxxxxx>
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Imho an error message in when YCbCR420 not allowed meets YCbCr420 only can be
a usefull bugfinding tool for cases of blackscreen on exotic configurations.

I'm unsure if this should be a warning instead.