drm/xe/configfs: Allow to select by class only

For a future configfs attribute, it's desirable to select by engine mask
only as the instance doesn't make sense.

Rename the function lookup_engine_mask() to lookup_engine_info() and
make it return the entry. This allows parse_engine() to still return an
item if the caller wants to allow parsing a class-only string like
"rcs", "bcs", "ccs", etc.

Reviewed-by: Raag Jadav <raag.jadav@intel.com>
Link: https://lore.kernel.org/r/20250916-wa-bb-cmds-v5-2-306bddbc15da@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>