drm/xe/reg_sr: Apply limit to register whitelisting

If RING_MAX_NONPRIV_SLOTS denotes the maximum number of whitelisting
slots, then it makes sense to refuse going above it.

v2:
  - Use xe_gt_err() instead of drm_err() for more detailed info in the
    error message. (Matt)

Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20230609143815.302540-3-gustavo.sousa@intel.com
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>