rust: drm: Add GPUVM Manager abstraction

Add a GPUVM Manager abstraction over the current C GPUVM framework.

This code is a thin abstraction, it does not preclude the development of a new
iteration of GPUVM entirely in Rust. This may be needed in order to properly
encode some features in idiomatic Rust.

For now, this thin abstraction should be enough to get early Rust DRM drivers
going.

Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
Co-developed-by: Janne Grunau <j@jannau.net>
Signed-off-by: Janne Grunau <j@jannau.net>
Signed-off-by: Asahi Lina <lina@asahilina.net>