drm/nouveau/core/memory: comptag allocation

nvkm_memory is going to be used by the upcoming mmu rework for the basic
representation of a memory allocation, as such, this commit adds support
for comptag allocation to nvkm_memory.

This is very simple for now, in that it requires comptags for the entire
memory allocation even if only certain ranges are compressed.

Support for tracking ranges will be added at a later date.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>