drm/vkms: Fix plane duplicate_state

We need to handle allocation failures and bail out. While at it, tune
the allocation failures down to debug level.

syzbot injected an allocation failure and then hit this WARN_ON.

Reported-by:  <syzbot+eb6e5365f23c02517dda@syzkaller.appspotmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Gustavo Padovan <gustavo.padovan@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181128101033.4840-1-daniel.vetter@ffwll.ch