drm/i915/cx0: split out mtl_get_cx0_buf_trans() to c10 and c20 variants

drm/i915/cx0: split out mtl_get_cx0_buf_trans() to c10 and c20 variants

The PHY is either c10 or c20, there's no need to check at runtime and
complicate the conditions in mtl_get_cx0_buf_trans().

While at it, replace the direct port clock check with
intel_dp_is_uhbr().

Cc: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241129114158.486418-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>