drm/nouveau/disp/dp: add support for eDP link rates

eDP 1.4 adds a table of link rates supported by the sink to DPCD, as
well as a LINK_RATE_SET register to select between the entries in it.

If present, we will use this data to generate our internal link rate
table rather than using the standard list based on MAX_LINK_RATE.

Some recent laptops report MAX_LINK_RATE=0, and require this support.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://gitlab.freedesktop.org/drm/nouveau/-/merge_requests/17