drm/amd/amdgpu: Add rev_id workaround logic for SRIOV setup

- When we are under SRIOV setup, the rev_id cannot be read
  properly. Therefore, we will return default value for it

Signed-off-by: Bokun Zhang <Bokun.Zhang@amd.com>
Reviewed-by: Monk Liu <monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>