drm/amdgpu: initialize cg/pg flags and external rev id for navi12

don't enable any cg/pg features yet.

v2: calculate external revision id from revision id so that we can
    differentiate navi12 A0 from A1 directly.
Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -596,6 +596,11 @@ static int nv_common_early_init(void *handle)
 			AMD_PG_SUPPORT_VCN_DPG;
 		adev->external_rev_id = adev->rev_id + 0x1; /* ??? */
 		break;
+	case CHIP_NAVI12:
+		adev->cg_flags = 0;
+		adev->pg_flags = 0;
+		adev->external_rev_id = adev->rev_id + 0xa;
+		break;
 	default:
 		/* FIXME: not supported yet */
 		return -EINVAL;