drm/vc4: tests: Use the correct return type in vc4_mock_atomic_add_output()#6928

pelwell merged 1 commit intoraspberrypi:rpi-6.12.yraspberrypi/linux:rpi-6.12.yfrom

mairacanal:vc4/downstream/fix-kunit-mockmairacanal/linux-rpi:vc4/downstream/fix-kunit-mockCopy head branch name to clipboard