st/nine: Fix second Multithreading issue with MANAGED buffers

st/nine: Fix second Multithreading issue with MANAGED buffers

Here is another threading issue with MANAGED buffers:

Thread 1: buffer creation
Thread 1: buffer lock
Thread 2: Draw call
Thread 1: writes data
Thread 1: Unlock

Without this patch, the buffer is initially dirty
and in the list of things to upload after its creation.
The draw call will then upload the data and unset the dirty flag,
and the Unlock won't trigger a second upload.

Fixes regression introduced by cc0114f30b587a10766ec212afb3ad356099ef23:
"st/nine: Implement Managed vertex/index buffers"

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