drm/radeon: properly set vm fragment size for TN/RL

commit a124d068 upstream.

Should be the same as cayman.  We don't use VM by default
on NI parts so this isn't critical.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>