rust: drm: fix `srctree/` links

rust: drm: fix `srctree/` links

These `srctree/` links pointed inside `linux/`, but they are directly
under `drm/`.

Thus fix them.

This cleans a future warning that will check our `srctree/` links.

Cc: stable@vger.kernel.org
Fixes: a98a73be9ee9 ("rust: drm: file: Add File abstraction")
Fixes: c284d3e42338 ("rust: drm: gem: Add GEM object abstraction")
Fixes: 07c9016085f9 ("rust: drm: add driver abstractions")
Fixes: 1e4b8896c0f3 ("rust: drm: add device abstraction")
Fixes: 9a69570682b1 ("rust: drm: ioctl: Add DRM ioctl abstraction")
Acked-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>