drm/amdgpu: Enable compilation under UML

The User-Mode Linux configuration is used by KUnit to execute kernel
results directly. This removes the need for a test machine or a virtual
machine. Therefore, as KUnit tests are being added to AMDGPU, it is
interesting to enable compilation under UML, as it eases running the tests
on CI and developers' machines.

Also, the use of UML is encouraged by the KUnit team [1], as it is considered
a better practice to write tests that run on UML to tests that only run
under a particular architecture.

[1] https://docs.kernel.org/dev-tools/kunit/usage.html#writing-tests-for-other-architectures



Signed-off-by: Maíra Canal <mairacanal@riseup.net>