drm/xe/svm: Migrate folios when possible

drm/xe/svm: Migrate folios when possible

The DMA mapping can now correspond to a folio (order > 0), so move
the iterator by the number of pages in the folio in order to migrate
all pages at once.

This requires forcing contiguous memory for SVM BOs, which greatly
simplifies the code and enables 2MB device page support, allowing a
major performance improvement. Negative effects like extra eviction
are unlikely as SVM BOs have a maximal size of 2MB.

v2:
- Improve commit message (Matthew Brost)
- Fix increment, chunk, assert match (Matthew Brost)

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250805140028.599361-7-francois.dugast@intel.com
Signed-off-by: Francois Dugast <francois.dugast@intel.com>