rust: drm: mm: Add DRM MM Range Allocator abstraction

drm_mm provides a simple range allocator, useful for managing virtual
address ranges. Add a Rust abstraction to expose this module to Rust
drivers.

The allocator will later be used by GPU drivers to allocate a range of VA
addresses when mapping a given BO object.

Signed-off-by: Asahi Lina <lina@asahilina.net>