rust: drm: gem: Add raw_dma_resv() function

For retrieving a pointer to the struct dma_resv for a given GEM object. We
also introduce it in a new trait, BaseObjectPrivate, which we automatically
implement for all gem objects and don't expose to users outside of the
crate.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Janne Grunau <j@jananu.net>
Tested-by: Janne Grunau <j@jannau.net>
Tested-by: Deborah Brouwer <deborah.brouwer@collabora.com>
Link: https://patch.msgid.link/20260316211646.650074-3-lyude@redhat.com
[ Fix incorrect reference in safety comment. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: Lyude

rust/kernel/drm/gem/mod.rs

@@ -215,6 +215,18 @@ pub trait BaseObject: IntoGEMObject {
 
 impl<T: IntoGEMObject> BaseObject for T {}
 
+/// Crate-private base operations shared by all GEM object classes.
+#[expect(unused)]
+pub(crate) trait BaseObjectPrivate: IntoGEMObject {
+    /// Return a pointer to this object's dma_resv.
+    fn raw_dma_resv(&self) -> *mut bindings::dma_resv {
+        // SAFETY: `self.as_raw()` always returns a valid pointer to the base DRM GEM object.
+        unsafe { (*self.as_raw()).resv }
+    }
+}
+
+impl<T: IntoGEMObject> BaseObjectPrivate for T {}
+
 /// A base GEM object.
 ///
 /// # Invariants