drm/atomic: Do not unset crtc when an encoder is stolen

While we steal the encoder away from the connector the connector may
be updated to use a different encoder.

Without this change if 2 connectors swap encoders one of them will
end up without a crtc.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1452160762-30487-5-git-send-email-maarten.lankhorst@linux.intel.com