drm/xe/mocs: Drop duplicate assignment of uc_index

The DG1 branch needlessly assigns uc_index twice.  Drop the second
instance.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>