drm/amdgpu: set metadata pointer to NULL after freeing.
Without this there was a double free of the metadata,
which ended up freeing the fd table for me here, and taking
out the machine more often than not.
I reproduced with X.org + modesetting DDX + latest llvm/mesa,
also required using dri3.
Cc: stable@vger.kernel.org Reviewed-by: Christian König <christian.koenig@amd.com> Signed-off-by: Dave Airlie <airlied@redhat.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>