drm/amdgpu: only check mmBIF_IOV_FUNC_IDENTIFIER on tonga/fiji

We only support SR-IOV on tonga/fiji.  Don't check this register
on other VI parts.

Fixes: 048765ad (amdgpu: fix asic initialization for virtualized environments (v2))
Reviewed-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org