drm/xe: Refactor __xe_exec_queue_create()

Split __xe_exec_queue_create() into two functions, alloc and init.

We have an issue in that exec_queue_user_extensions are applied too late.
In the case of USM properties, these need to be set prior to xe_lrc_init().
Refactor the logic here, so we can resolve this in follow-on. We only need
the xe_vm_lock held during the exec_queue_init function.

Signed-off-by: Brian Welty <brian.welty@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>