drm/amdgpu: add OOM fallback on PRT teardown (v2)

Don't assume kmalloc will always succeed.

v2: agd: rebase

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>