drm/amdgpu/gfx10: fix out-of-bound mqd_backup array access

Fixes: 0bb419c7 ("drm/amdgpu/gfx10: fix mqd backup/restore for gfx rings (v2)")
Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>