drm/amdgpu/ci: add mullins to default case for smc ucode

It's already covered by the default case, but add it for
consistency.

Reviewed-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>