rust: drm: don't pass the address of drm::Device to drm_dev_put()

rust: drm: don't pass the address of drm::Device to drm_dev_put()

In drm_dev_put() call in AlwaysRefCounted::dec_ref() we rely on struct
drm_device to be the first field in drm::Device, whereas everywhere
else we correctly obtain the address of the actual struct drm_device.

Analogous to the from_drm_device() helper, provide the
into_drm_device() helper in order to address this.

Fixes: 1e4b8896c0f3 ("rust: drm: add device abstraction")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250731154919.4132-5-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>