drm/nouveau: move io_reserve_lru handling into the driver v5

While working on TTM cleanups I've found that the io_reserve_lru used by
Nouveau is actually not working at all.

In general we should remove driver specific handling from the memory
management, so this patch moves the io_reserve_lru handling into Nouveau
instead.

v2: don't call ttm_bo_unmap_virtual in nouveau_ttm_io_mem_reserve
v3: rebased and use both base and offset in the check
v4: fix small typos and test the patch
v5: rebased and keep the mem.bus init in TTM.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/388643/