dri3: Only update number of back buffers in loader_dri3_get_buffers

dri3: Only update number of back buffers in loader_dri3_get_buffers

And only free no longer needed back buffers there as well.

We want to stick to the same back buffer throughout a frame, otherwise
we can run into various issues.

Bugzilla: https://bugs.freedesktop.org/105906
Bugzilla: https://bugs.freedesktop.org/106399
Fixes: 3160cb86aa92 "egl/x11: Re-allocate buffers if format is suboptimal"
Reported-by: Sergii Romantsov <sergii.romantsov@globallogic.com>
Tested-by: Eero Tamminen <eero.t.tamminen@intel.com>
Acked-by: Daniel Stone <daniels@collabora.com>