rusticl/mesa: make svm_migrate optional

It's just a hint and drivers might want to ignore implementing it for now.

Signed-off-by: Karol Herbst <git@karolherbst.de>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24092>

src/gallium/frontends/rusticl/mesa/pipe/context.rs

@@ -572,8 +572,6 @@ impl PipeContext {
                     to_device,
                     content_undefined,
                 );
-            } else {
-                panic!("svm_migrate not implemented but called!");
             }
         }
     }