drm/atomic: Always call steal_encoder, v2.

There's no need to have a separate function to get the crtc
which is stolen, this can already be found when actually
stealing the encoder.

drm_for_each_connector already checks for connection_mutex, so
use that macro now.

Changes since v1:
- Do not check for NULL crtc in connector_state,
  this may happen when a crtc is disabled and its encoder stolen.
- Because of this, use connector->state->crtc instead of conn_state->crtc.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1456996662-8704-4-git-send-email-maarten.lankhorst@linux.intel.com