drm/radeon: add some additional 6xx/7xx/EG register init

commit b866d1334ba2d544bc575d75357dea6bdcdc7f46 upstream.

- SMX_SAR_CTL0 needs to be programmed correctly to prevent
problems with memory exports in certain cases.
- VC_ENHANCE needs to be initialized on 6xx/7xx.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>