drm/nouveau/kms/nv50-: Just use drm_dp_dpcd_read() in nouveau_dp.c

Since this actually logs accesses, we should probably always be using
this imho…

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826182456.322681-4-lyude@redhat.com