drm/nouveau/kms: Use new drm_dp_read_mst_cap() helper for checking MST caps

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826182456.322681-11-lyude@redhat.com