drm/xe: Constify xe_dss_mask_group_ffs()

Due to how xe_dss_mask_t is implemented, the type is a pointer. Since
this is only used for looking up the bits, make it const so it can be
used together with a const gt passed around.

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