drm/virtio: Remove incorrect kfree()

The virtio_gpu_output is a member of struct virtio_gpu_device
and is not a dynamically-allocated chunk, so it's wrong to kfree() it.
Removing it fixes a memory corruption BUG() that can be triggered
when the virtio-gpu driver is removed.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190102175507.4653-1-ezequiel@collabora.com