drm/atomic: Clean up update_connector_routing.

connector_state->crtc can no longer be unset by accident,
so that check can be removed. The other code open-codes
drm_atomic_get_existing_crtc_state, so use that.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Resolve tiny conflict due to gcc warning fix on previous
patch.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>