drm/xe: Fix guc_exec_queue_set_priority

We need to set q->priority prior to calling guc_exec_queue_add_msg() as
that will call init_policies() and sets the scheduling properties to those
stored in the exec_queue.

Fixes: dd08ebf6

 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
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>
(cherry picked from commit b16483f9

)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>