loader/dri3: Only allocate additional buffers if needed

loader/dri3: Only allocate additional buffers if needed

Previously, we would always allocate 3 buffers for page flipping. But 2
buffers can suffice for clients which always wait for buffer swaps to
complete before starting a new frame.

Therefore, keep track of the maximum number of buffers separately from
the current number, and only bump the latter if both current buffers are
busy.

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>