drm/radeon: fix cursor corruption on DCE6 and newer

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>
Cc: stable@vger.kernel.org