loader/dri3: Keep current number of back buffers if frame was skipped

loader/dri3: Keep current number of back buffers if frame was skipped

We'd previously take the copy path. If we were actually flipping (in
which case skipped frames are more likely to occur), we'd ping-pong
between a smaller and larger number of back buffers, and frame-rate
could vary / take a dip due to the buffer management overhead.

While I'm not sure this is actually possible to hit at this point, it
definitely will be with the next change.

Cc: mesa-stable
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7033>