Create a struct to represent MDP5 Layer Mixer instances. This will
eventually allow us to detach CRTCs from the Layer Mixers, and
generally clean things up a bit.

This is very similar to how hwpipes were previously abstracted away
from drm planes.
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>