drm/amdgpu/cgs: remove import_gpu_mem

It was added for completeness, but we don't have any users
for it yet.  Daniel noted that it may be racy. Remove it.

Change-Id: I5f5546f8911a4f294008a62dc86a73f3face38d1
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>