rust: drm: ioctl: Add DRM ioctl abstraction

DRM drivers need to be able to declare which driver-specific ioctls they
support. Add an abstraction implementing the required types and a helper
macro to generate the ioctl definition inside the DRM driver.

Note that this macro is not usable until further bits of the abstraction
are in place (but it will not fail to compile on its own, if not called).

Signed-off-by: Asahi Lina <lina@asahilina.net>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://lore.kernel.org/r/20250410235546.43736-3-dakr@kernel.org
[ MISC fixes
    * wrap raw_data in Opaque to avoid UB when creating a reference
    * fix IOCTL sample declaration
    * fix safety comment of IOCTL argument
    * original source archive: https://archive.is/LqHDQ



  - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>