drm/kms: add explicit encoder disable function and detach harder.

For shared tv-out and VGA encoders, we really need to know if
the encoder is just being switched off temporarily in blanking
or if we are really disabling it hard.

Also we need to try harder to disconnect encoders from unused
connectors so we can share more efficently.

(shared encoders stuff is coming in radeon tv-out support)

Signed-off-by: Dave Airlie <airlied@redhat.com>