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>

drivers/gpu/drm/amd/amdgpu/ci_dpm.c

@@ -5779,6 +5779,7 @@ static int ci_dpm_init_microcode(struct amdgpu_device *adev)
 		break;
 	case CHIP_KAVERI:
 	case CHIP_KABINI:
+	case CHIP_MULLINS:
 	default: BUG();
 	}