drm/gpusvm: Introduce a function to scan the current migration state

drm/gpusvm: Introduce a function to scan the current migration state

With multi-device we are much more likely to have multiple
drm-gpusvm ranges pointing to the same struct mm range.

To avoid calling into drm_pagemap_populate_mm(), which is always
very costly, introduce a much less costly drm_gpusvm function,
drm_gpusvm_scan_mm() to scan the current migration state.
The device fault-handler and prefetcher can use this function to
determine whether migration is really necessary.

There are a couple of performance improvements that can be done
for this function if it turns out to be too costly. Those are
documented in the code.

v3:
- New patch.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> # For merging through drm-xe.
Link: https://patch.msgid.link/20251219113320.183860-21-thomas.hellstrom@linux.intel.com