drm/radeon/kms: Convert RS600 to new init path

New init path allow to simply asic initialization and make easier
to trace what happen on each different asic. We are removing most
callback. Do a massive RS600 register cleanup to clarify RS600
register, we are still bit fuzy on some register and waiting for
more informations. I don't have hw to test, so this patch is a
best effort to not break anythings and to try to improve things.

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