rust: device: fix unresolved link to drm::Device

rust: device: fix unresolved link to drm::Device

drm::Device is only available when CONFIG_DRM=y, which we have to
consider for intra-doc links, otherwise the rustdoc make target produces
the following warning.

>> warning: unresolved link to `kernel::drm::Device`
   --> rust/kernel/device.rs:154:22
   |
   154 | /// [`drm::Device`]: kernel::drm::Device
   |                      ^^^^^^^^^^^^^^^^^^^ no item named `drm` in module `kernel`
   |
   = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

Fix this by making the intra-doc link conditional on CONFIG_DRM being enabled.

Fixes: d6e26c1ae4a6 ("device: rust: expand documentation for Device")
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202508261644.9LclwUgt-lkp@intel.com/
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250829195745.31174-1-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>