drm/nouveau/kms/nv50-: Use downstream DP clock limits for mode validation

This adds support for querying the maximum clock rate of a downstream
port on a DisplayPort connection. Generally, downstream ports refer to
active dongles which can have their own pixel clock limits.

Note as well, we also start marking the connector as disconnected if we
can't read the DPCD, since we wouldn't be able to do anything without
DPCD access anyway.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826182456.322681-15-lyude@redhat.com