drm/radeon/kms: Convert R520 to new init path and associated cleanup

Convert the r520 asic support to new init path, change are smaller than
previous one as most of the architecture is now in place and more code
sharing can happen btw various asics.

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