drm/radeon: fix cursor corruption on DCE6 and newer

commit e521a29014794d139cca46396d1af8faf1295a26 upstream.

Aruba and newer gpu does not need the avivo cursor work around,
quite the opposite this work around lead to corruption.

agd5f: check DCE6 rather than ARUBA since the issue is DCE
version specific rather than family specific.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>