drm/virtio: Allow importing prime buffers when 3D is enabled

drm/virtio: Allow importing prime buffers when 3D is enabled

This functionality was added for using a KMS-only virtgpu with a physical
(or SR-IOV) headless GPU in passthrough, but it should not be restricted
to KMS-only mode. It can be used with cross-domain to pass guest memfds
to the host compositor with zero copies (using udmabuf on both sides).

Drop the check for the absence of virgl_3d to allow for more use cases.

Fixes: ca77f27a2665 ("drm/virtio: Import prime buffers from other devices as guest blobs")
Signed-off-by: Val Packett <val@invisiblethingslab.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patch.msgid.link/20251210154755.1119861-2-val@invisiblethingslab.com