drm/xe/configfs: Fix engine class parsing

drm/xe/configfs: Fix engine class parsing

If mask is NULL, only the engine class should be accepted, so the
pattern string should be completely parsed. This should fix passing e.g.
rcs0 to ctx_restore_post_bb when it's only expecting the engine class.

Reported-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Closes: https://lore.kernel.org/r/20250922155544.67712-1-jonathan.cavitt@intel.com
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/aNJKnrCQmL9xS9Gv@stanley.mountain
Fixes: e2a9854d806e ("drm/xe/configfs: Allow to select by class only")
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Raag Jadav <raag.jadav@intel.com>
Link: https://lore.kernel.org/r/20250924152709.659483-3-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>