drm/amdgpu: remove WARN_ON when VM isn't found v2

It can easily be that the VM is already destroyed when this runs.

v2: fix test inversion

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