loader/dri3: Also wait for front buffer fence if we triggered it

In that case, we have to wait for the fence to synchronize with the
corresponding drawing we triggered in the X server.

Fixes incorrect display with the i965 driver and some applications, e.g.
solvespace.

Bugzilla: https://bugs.freedesktop.org/108097


Fixes: aefac10f "loader/dri3: Only wait for back buffer fences in
                     dri3_get_buffer"
Tested-by: Sergii Romantsov <sergii.romantsov@globallogic.com>
(cherry picked from commit c20ba1be)