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

In the Linux kernel, the following vulnerability has been resolved:

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.

(cherry picked from commit 04f4a70a183a688a60fe3882d6e4236ea02cfc67)