drm/amdgpu: Revert "remove the userptr rmn->lock"

This reverts commit c02196834456f2d5fad334088b70e98ce4967c34.

In the meantime we moved get_user_pages() outside of the reservation lock,
so that shouldn't be an issue any more

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>