drm/amdgpu: remove VM shadow WARN_ONs

Printing a warning into the logs that we will certainly run into a BUG() is
completely nonsense, the BUG() is more than noisy enough.

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