drm/xe/bo: consider bo->flags in xe_bo_migrate()

For VRAM allocations the bo->flags can control some characteristics of
the underlying memory, like whether it needs to be contiguous, and in
the future whether it needs to be in the CPU visible portion. Rather use
add_vram() in xe_bo_migrate() which should take care of such things for
us.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>