loader/dri3: Overhaul dri3_update_num_back

loader/dri3: Overhaul dri3_update_num_back

Always use 3 buffers when flipping. With only 2 buffers, we have to wait
for a flip to complete (which takes non-0 time even with asynchronous
flips) before we can start working on the next frame. We were previously
only using 2 buffers for flipping if the X server supports asynchronous
flips, even when we're not using asynchronous flips. This could result
in bad performance (the referenced bug report is an extreme case, where
the inter-frame stalls were preventing the GPU from reaching its maximum
clocks).

I couldn't measure any performance boost using 4 buffers with flipping.
Performance actually seemed to go down slightly, but that might have
been just noise.

Without flipping, a single back buffer is enough for swap interval 0,
but we need to use 2 back buffers when the swap interval is non-0,
otherwise we have to wait for the swap interval to pass before we can
start working on the next frame. This condition was previously reversed.

Cc: "12.0 11.2" <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97260
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: Eric Anholt <eric@anholt.net>