drm/i915: Extract intel_dp_has_dsc()

Extract a helper to check whether the source+sink combo
supports DSC. That basic check is needed both during mode
validation and compute config. We'll also need to add extra
checks to both places, so having a single place for it is nicer.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240517145356.26103-3-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>