drm/radeon: fix typo in trinity tiling setup

commit 1f73cca799d29df80de3e8f1f1c488485467577a upstream.

Using the wrong union.

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