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: 8c2d61e0 ("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>