drm/xe/hw_engine_group: Fix potential leak

If we fail to allocate a workqueue we will leak kzalloc'ed group
object since it was designed to be kfree'ed in the drmm cleanup
action, but we didn't have a chance to register this action yet.

To avoid this leak allocate a group object using drmm_kzalloc()
and start using predefined drmm action to release the workqueue.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Francois Dugast <francois.dugast@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250627184143.1480-1-michal.wajdeczko@intel.com