drm/radeon/kms: rs400/480 MC setup is different than r300.

Boot testing on my rs480 laptop found the MC idle never happened
on startup, a quick check with AMD found the idle bit is in a different
place on the rs4xx than r300.

Implement a new rs400 mc idle function to fix this.

Signed-off-by: Dave Airlie <airlied@redhat.com>