drm/virtio: Use vmalloc for command buffer allocations.

Userspace requested command buffer allocations could be too large
to make as a contiguous allocation.  Use vmalloc if necessary to
satisfy those allocations.

Signed-off-by: David Riley <davidriley@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190911181403.40909-3-davidriley@chromium.org


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>