drm: Make the connector .detect() callback optional

Many drivers (21 to be exact) create connectors that are always
connected (for instance to an LVDS or DSI panel). Instead of forcing
them to implement a dummy .detect() handler, make the callback optional
and consider the connector as always connected in that case.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Vincent Abriou <vincent.abriou@st.com>
Acked-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
[seanpaul fixed small conflict in rcar-du/rcar_du_lvdscon.c]
Signed-off-by: Sean Paul <seanpaul@chromium.org>

include/drm/drm_connector.h

@@ -262,6 +262,9 @@ struct drm_connector_funcs {
 	 * connector due to a user request. force can be used by the driver to
 	 * avoid expensive, destructive operations during automated probing.
 	 *
+	 * This callback is optional, if not implemented the connector will be
+	 * considered as always being attached.
+	 *
 	 * FIXME:
 	 *
 	 * Note that this hook is only called by the probe helper. It's not in