For MST we need to be able to pick front end encoders
separate from backend, but only for MST, so we need to
make the encoder picking interface smarter.

v2: agd5f: squash in:
drm/radeon: release digital encoder before asking for new one
Reported-by: NDieter Nützel <Dieter@nuetzel-hh.de>
Signed-off-by: NDave Airlie <airlied@redhat.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>