drm/xe: Only use reserved BCS instances for usm migrate exec queue

The GuC context scheduling queue is 2 entires deep, thus it is possible
for a migration job to be stuck behind a fault if migration exec queue
shares engines with user jobs. This can deadlock as the migrate exec
queue is required to service page faults. Avoid deadlock by only using
reserved BCS instances for usm migrate exec queue.

Fixes: a043fbab

 ("drm/xe/pvc: Use fast copy engines as migrate engine on PVC")
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240415190453.696553-2-matthew.brost@intel.com
Reviewed-by: Brian Welty <brian.welty@intel.com>
(cherry picked from commit 04f4a70a

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