Now that everything's atomic, checking encoder->base.crtc is enough.
This function doesn't have the locks to dereference crtc->state, but
stealing an encoder bound to any crtc is probably enough reason to warn.

Changes since v1:
- Commit message.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>