drm/connector: Share with non-atomic drivers the function to get the single encoder

This 3 non-atomic drivers all have the same function getting the
only encoder available in the connector, also atomic drivers have
this fallback. So moving it a common place and sharing between atomic
and non-atomic drivers.

While at it I also removed the mention of
drm_atomic_helper_best_encoder() that was renamed in
commit 297e30b5 ("drm/atomic-helper: Unexport
drm_atomic_helper_best_encoder").

v3: moving drm_connector_get_single_encoder to drm_kms_helper module

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: dri-devel@lists.freedesktop.org
Cc: intel-gfx@lists.freedesktop.org
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190913232857.389834-1-jose.souza@intel.com