drm/vc4: tests: Return the allocated output

Some tests will need to retrieve the output that was just allocated by
vc4_mock_atomic_add_output().

Instead of making them look them up in the DRM device, we can simply
make vc4_mock_atomic_add_output() return an error pointer that holds the
allocated output instead of the error code.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>