drm/amdgpu: Wipe all VRAM on free when RAS is enabled

On GPUs with RAS, poison can propagate between processes if VRAM is not
cleared when it is freed or allocated. The reason is, that not all write
accesses clear RAS poison. 32-byte writes by the SDMA engine do clear RAS
poison. Clearing memory in the background when it is freed should avoid
major performance impact. KFD has been doing this already for a long time.

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