drm/nouveau/kms/nv50-: Add support for DP_SINK_COUNT

This is another bit that we never implemented for nouveau: dongle
detection. When a "dongle", e.g. an active display adaptor, is hooked up
to the system and causes an HPD to be fired, we don't actually know
whether or not there's anything plugged into the dongle without checking
the sink count. As a result, plugging in a dongle without anything
plugged into it currently results in a bogus EDID retrieval error in the kernel log.

Additionally, most dongles won't send another long HPD signal if the
user suddenly plugs something in, they'll only send a short HPD IRQ with
the expectation that the source will check the sink count and reprobe
the connector if it's changed - something we don't actually do. As a
result, nothing will happen if the user plugs the dongle in before
plugging something into the dongle.

So, let's fix this by checking the sink count in both
nouveau_dp_probe_dpcd() and nouveau_dp_irq(), and reprobing the
connector if things change.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826182456.322681-18-lyude@redhat.com