drm/amdgpu/gfx10: fix mqd backup/restore for gfx rings (v2)

1. no need to allocate an extra member for 'mqd_backup' array
2. backup/restore mqd to/from the correct 'mqd_backup' array slot

v2: warning fix (Alex)

Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>