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>