drm/vc4: dsi: Only register our component once a DSI device is attached

If the DSI driver is the last to probe, component_add will try to run all
the bind callbacks straight away and return the error code.

However, since we depend on a power domain, we're pretty much guaranteed to
be in that case on the BCM2711, and are just lucky on the previous SoCs
since the v3d also depends on that power domain and is further in the probe
order.

In that case, the DSI host will not stick around in the system: the DSI
bind callback will be executed, will not find any DSI device attached and
will return EPROBE_DEFER, and we will then remove the DSI host and ask to
be probed later on.

But since that host doesn't stick around, DSI devices like the RaspberryPi
touchscreen whose probe is not linked to the DSI host (unlike the usual DSI
devices that will be probed through the call to mipi_dsi_host_register)
cannot attach to the DSI host, and we thus end up in a situation where the
DSI host cannot probe because the panel hasn't probed yet, and the panel
cannot probe because the DSI host hasn't yet.

In order to break this cycle, let's wait until there's a DSI device that
attaches to the DSI host to register the component and allow to progress
further.

Acked-by: Eric Anholt <eric@anholt.net>
Suggested-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200707101912.571531-1-maxime@cerno.tech