drm/nouveau/core/memory: change map interface to support upcoming mmu changes

Map flags (access, kind, etc) are currently defined in either the VMA,
or the memory object, which turns out to not be ideal for things like
suballocated buffers, etc.

These will become per-map flags instead, so we need to support passing
these arguments in nvkm_memory_map().

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