drm/i915/display: Restructure output format computation for better expandability

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.

A slight change in behaviour occurs with this patch: 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@tuxedocomputers.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210510133349.14491-3-wse@tuxedocomputers.com