loader/dri3: Only wait for back buffer fences in dri3_get_buffer

We don't need to wait before drawing to the fake front buffer, as front
buffer rendering by definition is allowed to produce artifacts.

Fixes hangs in some cases when re-using the fake front buffer, due to it
still being busy (i.e. in use for presentation).

Cc: mesa-stable@lists.freedesktop.org
Bugzilla: https://bugs.freedesktop.org/106404
Bugzilla: https://bugs.freedesktop.org/107757
Tested-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>