drm/radeon: fix bank tiling parameters on cayman

commit 5b23c9045a8b61352986270b2d109edf5085e113 upstream.

Handle the 16 bank case.

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