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: [e-mail]
Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/amdgpu_object.c

···
 	if (!metadata_size) {
 		if (bo->metadata_size) {
 			kfree(bo->metadata);
+			bo->metadata = NULL;
 			bo->metadata_size = 0;
 		}
 		return 0;