drm/xe: Ensure fixed_slice_mode gets set after ccs_mode change

The RCU_MODE_FIXED_SLICE_CCS_MODE setting is not getting invoked
in the gt reset path after the ccs_mode setting by the user.
Add it to engine register update list (in hw_engine_setup_default_state())
which ensures it gets set in the gt reset and engine reset paths.

v2: Add register update to engine list to ensure it gets updated
after engine reset also.

Fixes: 0d97ecce ("drm/xe: Enable Fixed CCS mode setting")
Cc: stable@vger.kernel.org
Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250327185604.18230-1-niranjana.vishwanathapura@intel.com