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