drm/virtio: Set segment size for virtio_gpu device

Set the segment size of the virtio_gpu device to the value
used by the drm helpers when allocating sg lists to fix the
following complaint from DMA_API debug code:

DMA-API: virtio-pci 0000:07:00.0: mapping sg segment longer than
device claims to support [len=262144] [max=65536]

Cc: stable@vger.kernel.org
Tested-by: Zhenyu Zhang <zhenyzha@redhat.com>
Acked-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Sebastian Ott <sebott@redhat.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7258a4cc-da16-5c34-a042-2a23ee396d56@redhat.com