drm/amdgpu/gmc7: add missing mullins case

Looks like this got missed when we ported the code from radeon.
Reviewed-by: NEdward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c

@@ -144,6 +144,7 @@ static int gmc_v7_0_init_microcode(struct amdgpu_device *adev)
 		break;
 	case CHIP_KAVERI:
 	case CHIP_KABINI:
+	case CHIP_MULLINS:
 		return 0;
 	default: BUG();
 	}