drm/xe/svm: Ensure data will be migrated to system if indicated by madvise.

drm/xe/svm: Ensure data will be migrated to system if indicated by madvise.

If the location madvise() is set to
DRM_XE_PREFERRED_LOC_DEFAULT_SYSTEM, the drm_pagemap in the
SVM gpu fault handler will be set to NULL. However there is nothing
that explicitly migrates the data to system if it is already present
in device memory.

In that case, set the device memory owner to NULL to ensure
data gets properly migrated to system on page-fault.

v2:
- Remove redundant dpagemap assignment (Himal Prasad Ghimiray)

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com> #v1
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://lore.kernel.org/r/20251010104149.72783-2-thomas.hellstrom@linux.intel.com
Fixes: 10aa5c806030 ("drm/gpusvm, drm/xe: Fix userptr to not allow device private pages")