drm/radeon/kms: move active_device setup into encoder fixup()

radeon_encoder->active_device defines the active routing
between the encoder and connector.  The encoder fixup and
dpms functions need to know the active_device to function
properly.  Setting active_device in the prepare hook was
too late in some cases.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>