rust: drm: device: drop_in_place() the drm::Device in release()

In drm::Device::new() we allocate with __drm_dev_alloc() and return an
ARef<drm::Device>.

When the reference count of the drm::Device falls to zero, the C code
automatically calls drm_dev_release(), which eventually frees the memory
allocated in drm::Device::new().

However, due to that, drm::Device::drop() is never called. As a result
the destructor of the user's private data, i.e. drm::Device::data is
never called. Hence, fix this by calling drop_in_place() from the DRM
device's release callback.

Fixes: 1e4b8896 ("rust: drm: add device abstraction")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250629153747.72536-1-dakr@kernel.org