drm/virtio: Ensure cached capset entries are valid before copying.

drm/virtio: Ensure cached capset entries are valid before copying.

virtio_gpu_get_caps_ioctl could return success with invalid data if a
second caller to the function occurred after the entry was created in
virtio_gpu_cmd_get_capset but prior to the virtio_gpu_cmd_capset_cb
callback being called.  This could leak contents of memory as well
since the caps_cache allocation is done without zeroing.

Signed-off-by: David Riley <davidriley@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190605234423.11348-1-davidriley@chromium.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>