drm/amdgpu/gmc10: free stolen memory in late_init

We don't need to store the pre-OS console memory after
the driver has loaded so free it.

Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>