drm/radeon/kms: fix tile_config value reported to userspace on cayman.

cayman is reporting the wrong tile config value to userspace, this
causes piglit mipmap generation tests to fail.

Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>