drm/xe/reg_sr: Save errors for kunit integration

When there's an entry that is dropped when xe_reg_sr_add(), there's
not much we can do other than reporting the error - it's for certain a
driver issue or conflicting workarounds/tunings. Save the number of
errors to be used later by kunit to report where it happens.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20230401085151.1786204-6-lucas.demarchi@intel.com