drm/amdgpu: The VI specific EXE bit should only apply to GMC v8.0 above

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Ken Wang <Qingqing.Wang@amd.com>
Cc: stable@vger.kernel.org