drm/amdgpu: freeing pinned objects is illegal now

We want to drop support in TTM for this.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Leo Liu <leo.liu@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210415084730.2057-2-christian.koenig@amd.com