drm/i915: Don't deref NULL crtc in intel_get_pipe_from_connector()

If the connector would have an encoder but the encoder didn't have a
crtc we might dereference a NULL crtc here. I suppose that should never
happen due to intel_sanitize_encoder(), but let's be a bit paranoid
print a warning if we ever hit this and return INVALID_PIPE to the
caller.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -11999,7 +11999,7 @@ enum pipe intel_get_pipe_from_connector(struct intel_connector *connector)
 
 	WARN_ON(!drm_modeset_is_locked(&dev->mode_config.connection_mutex));
 
-	if (!encoder)
+	if (!encoder || WARN_ON(!encoder->crtc))
 		return INVALID_PIPE;
 
 	return to_intel_crtc(encoder->crtc)->pipe;