drm/amdgpu: add check before enabling/disabling broadcast mode

When security violation from new vbios happens, data fabric is
risky to stop working. So prevent the direct access to DF
mmFabricConfigAccessControl from the new vbios and onwards.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>