drm/radeon: fix bank tiling parameters on evergreen

commit c8d15edc17d836686d1f071e564800e1a2724fa6 upstream.

Handle the 16 bank case.

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