drm/i915/dp: Skip computing a non-DSC link config if DSC is needed

Computing the non-DSC mode link config is redundant once it's determined
that DSC will be needed, so skip computing it. In a follow-up patch this
simplifies setting the link limits which are dependent on the DSC vs.
non-DSC mode.

While at it sanitize the debug print about the MST DSC fallback path,
making it similar to the SST DSC one.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230921195159.2646027-4-imre.deak@intel.com