drm/amdgpu/gmc10: remove dead code

Leftover from bring up.  We look up the actual pre-OS memory usage
value later in the same function.

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>