BACKPORT: 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>

Bug: https://crbug.com/993452


Change-Id: I253a9eb22fdee4836d48abecbe836ee5369e6526
(cherry picked from commit e1218b8c0cc1f8108be67ba3783d63eb4a50d792)
[kaiyili: Resolved minor conflict in drivers/gpu/drm/virtio/virtgpu_vq.c ]
Signed-off-by: Kaiyi Li <kaiyili@google.com>