nouveau: add an ioctl to report vram usage

[ Upstream commit 72fa02fd ]

This reports the currently used vram allocations.

userspace using this has been proposed for nvk, but
it's a rather trivial uapi addition.

Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Stable-dep-of: aed9a1a4 ("drm/nouveau: use tile_mode and pte_kind for VM_BIND bo allocations")
Signed-off-by: Sasha Levin <sashal@kernel.org>