drm/xe/migrate: don't misalign current bytes

If current bytes exceeds the max copy size, ensure the clamped size
still accounts for the XE_CACHELINE_BYTES alignment, otherwise we
trigger the assert in xe_migrate_vram with the size now being out of
alignment.

Fixes: 8c2d61e ("drm/xe/migrate: don't overflow max copy size")
Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6212
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Stuart Summers <stuart.summers@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20251010162020.190962-2-matthew.auld@intel.com
(cherry picked from commit 641bcf8731d21b56760e3646a39a65f471e9efd1)
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>

Author: matt-auld

drivers/gpu/drm/xe/xe_migrate.c

@@ -2113,7 +2113,9 @@ int xe_migrate_access_memory(struct xe_migrate *m, struct xe_bo *bo,
 		if (current_bytes & ~PAGE_MASK) {
 			int pitch = 4;
 
-			current_bytes = min_t(int, current_bytes, S16_MAX * pitch);
+			current_bytes = min_t(int, current_bytes,
+					      round_down(S16_MAX * pitch,
+							 XE_CACHELINE_BYTES));
 		}
 
 		__fence = xe_migrate_vram(m, current_bytes,