drm/pagemap: Disable device-to-device migration

drm/pagemap: Disable device-to-device migration

Device-to-device migration is causing xe_exec_system_allocator --r
*race*no* to intermittently fail with engine resets and a kernel hang on
a page lock. This should work but is clearly buggy somewhere. Disable
device-to-device migration in the interim until the issue can be
root-caused.

The only downside of disabling device-to-device migration is that memory
will bounce through system memory during migration. However, this path
should be rare, as it only occurs when madvise attributes are changed or
atomics are used.

Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Fixes: ec265e1f1cfc ("drm/pagemap: Support source migration over interconnect")
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patch.msgid.link/20260107182716.2236607-3-matthew.brost@intel.com