drm/nouveau/core/mm: have users explicitly define heap identifiers

Different sections of VRAM may have different properties (ie. can't be used
for compression/display, can't be mapped, etc).

We currently already support this, but it's a bit magic.  This change makes
it more obvious where we're allocating from.

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