drm/amdgpu/umsch: add suspend and resume callback

Add missing IP callbacks.

Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Veerabadhran Gopalakrishnan <Veerabadhran.Gopalakrishnan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>