drm/amdgpu: Zero-initialize mqd backup memory

Zero-initialize mqd backup memory, otherwise the check for
'already-backed-up' could go wrong.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>