drm/i915: rename VLV IOSF SB ports parameter to unit_mask
Clarify what the parameter is about through proper naming.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/b1be9a4089dfbf48ea1434944a4841d2ac4c4acc.1747061743.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>