drm/amdgpu: set external rev id for raven2

It's different from raven1.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>