st/nine: Immediately upload user provided textures

Fixes regression caused by
42d672fa
st/nine: Bind src not dst in nine_context_box_upload

Before that patch, for user provided textures,
when the texture was destroyed, the safety
check for pending uploads, which according to
the code "Following condition cannot happen currently",
was flushing the queue and thus triggering the upload.

After the patch, the texture destruction was delayed after
the upload. However the user frees the texture buffer,
as it thinks the texture released.

Instead of reverting the faulty patch,
this patch instead flushes the csmt queue right away
after queuing the upload for this type of textures.
This is more future-proof, as we may want to bind the
surface for other reasons in the future.

Signed-off-by: Axel Davy <davyaxel0@gmail.com>
Cc: 18.3 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit d7433c22)