Pull the "does joiner need DSC?" check into a helper. MST will
want to use this too at some point.
Tested-by: Vidya Srinivas <vidya.srinivas@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240404213441.17637-9-ville.syrjala@linux.intel.com