baikal_vdu_drm module does not get loaded automatically

[asheplyakov]

As of now one need to manually modprobe baikal_vdu_drm (possibly several times) or compile the thing into the kernel (not acceptable for a kernel which is supposed to run on several/many SoCs).
The cause of such misbehavior is that hdmi transmitter might not be ready during baikal_vdu_drm_probe. Linux has got a technique to deal with such situations: aggregate drivers. Rewrite baikal_vdu_drm as an aggregate driver (similarly to drivers/gpu/drm/meson)