drm/virtio: fix a wait_event condition

outcnt may be greater than 1 since commit e1218b8c ("drm/virtio:
Use vmalloc for command buffer allocations.").

Fixes: e1218b8c ("drm/virtio: Use vmalloc for command buffer allocations.")
Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Cc: David Riley <davidriley@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20200205181955.202485-2-olvaffe@gmail.com


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