drm/virtio: fix OOB in virtio_gpu_object_create

After commit f651c8b0

 ("drm/virtio: factor out the sg_table from
virtio_gpu_object"), virtio_gpu_create_object allocates too small space
to fit everything in. It is because it allocates struct
virtio_gpu_object, but should allocate a newly added struct
virtio_gpu_object_shmem which has 2 more members.

So fix that by using correct type in virtio_gpu_create_object.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: http://patchwork.freedesktop.org/patch/msgid/20200319100421.16267-1-jslaby@suse.cz
Fixes: f651c8b0

 ("drm/virtio: factor out the sg_table from virtio_gpu_object")
Cc: Gurchetan Singh <gurchetansingh@chromium.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>