drm/gpusvm, drm/xe: Allow mixed mappings for userptr

drm/gpusvm, drm/xe: Allow mixed mappings for userptr

Compute kernels often issue memory copies immediately after completion.
If the memory being copied is an SVM pointer that was faulted into the
device and then bound via userptr, it is undesirable to move that
memory. Worse, if userptr is mixed between system and device memory, the
bind operation may be rejected.

Xe already has the necessary plumbing to support userptr with mixed
mappings. This update modifies GPUSVM's get_pages to correctly locate
pages in such mixed mapping scenarios.

v2:
- Rebase (Thomas Hellström)
v3:
- Remove Fixes tag.
v4:
- Break out from series since the other patch was merged.
- Update patch subject, ensure dri-devel and Maarten are CC'd.

Cc: Maarten Lankhorst <maarten.lankhorst@intel.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://lore.kernel.org/r/20251015120320.176338-1-thomas.hellstrom@linux.intel.com
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>