drm/crtc-helper: drm_connector_get_single_encoder prototype is missing

Include drm_crtc_helper_internal.h to provide drm_connector_get_single_encoder
prototype.
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191119125805.4266-1-benjamin.gaignard@st.com

drivers/gpu/drm/drm_crtc_helper.c

@@ -48,6 +48,8 @@
 #include <drm/drm_print.h>
 #include <drm/drm_vblank.h>
 
+#include "drm_crtc_helper_internal.h"
+
 /**
  * DOC: overview
  *