Add infrastructure to keep track of GPU virtual address (VA) mappings
with a decicated VA space manager implementation.

New UAPIs, motivated by Vulkan sparse memory bindings graphics drivers
start implementing, allow userspace applications to request multiple and
arbitrary GPU VA mappings of buffer objects. The DRM GPU VA manager is
intended to serve the following purposes in this context.

1) Provide infrastructure to track GPU VA allocations and mappings,
   using an interval tree (RB-tree).

2) Generically connect GPU VA mappings to their backing buffers, in
   particular DRM GEM objects.

3) Provide a common implementation to perform more complex mapping
   operations on the GPU VA space. In particular splitting and merging
   of GPU VA mappings, e.g. for intersecting mapping requests or partial
   unmap requests.

Acked-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Tested-by: Matthew Brost <matthew.brost@intel.com>
Tested-by: Donald Robson <donald.robson@imgtec.com>
Suggested-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230720001443.2380-2-dakr@redhat.com