drm/radeon: SI tiling fixes for display

- Use the correct union for getting the tiling info
- Properly init the PIPE_CONFIG field for SI

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>