drm/radeon: fix gpu_init on si

- Properly set up the RBs
- Properly set up the SPI
- Properly set up gb_addr_config

This should fix rendering issues on certain cards.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>