drm/amdgpu: check df_funcs and its callback pointers
in case they are not avaiable in early phase
Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Le Ma <Le.Ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
authored by