drm/nouveau/kms/nv50-: Use drm_dp_dpcd_(readb|writeb)() in nv50_sor_disable()

Just use drm_dp_dpcd_(readb|writeb)() so we get automatic DPCD logging

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826182456.322681-8-lyude@redhat.com