st/nine: Fix Multithreading issue with MANAGED buffers

st/nine: Fix Multithreading issue with MANAGED buffers

d3d calls are protected by mutexes, however if app is doing in
two threads:

Thread 1: buffer Lock
Thread 2: Draw call
Thread 1: writes data
Thread 1: Unlock

Then before this patch, the Draw call would begin to upload
the buffer.

Solves this by moving the moment we add the buffer to the queue
of things to upload (We move it from Lock time to Unlock time).

Cc: "11.2" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Axel Davy <axel.davy@ens.fr>