drm/i915: Make intel_get_pipe_from_connector atomic

Drive-by fixup while looking at all the connector_list walkers -
holding connection_mutex does actually _not_ give you locking to look
at the legacy drm_connector->encoder->crtc pointer chain. That one is
solely owned by the atomic commit workers. Instead we must inspect the
atomic state.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170301095226.30584-4-daniel.vetter@ffwll.ch