Pretty straightforward changes, merely mimicking dm-related code,
which was already essentially using either Mesh or BMesh data to
build the PBVH...
Note that we "lose" the subsurf (a.k.a. grid) PBVH case here, but that
one was already dead code in current blender2.8, since final dm is
always a cddm built from evaluated mesh.
Proper fix is pending new code for subsurf/multires area.
3e86bb2d0b