drm/amdgpu/gmc7: add missing mullins case

BugLink: http://bugs.launchpad.net/bugs/1615620

commit 7f555c8e upstream.

Looks like this got missed when we ported the code from radeon.
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

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

@@ -167,6 +167,7 @@ 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();
 	}