vbo: lower VBO_SAVE_BUFFER_SIZE to avoid large VRAM usage

vbo: lower VBO_SAVE_BUFFER_SIZE to avoid large VRAM usage

The ideal case for performance is to have a single buffer for
all display list. The caveat is that large buffers are less
likely to be freed because they're refcounted: it only takes
1 user (diplay list) to keep it in VRAM.

This lowers VRAM usage when replaying the trace attached
of the trace attached to !6140 from 5.5 GB to about 1.8 GB.

Viewperf snx performance isn't affected.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6140
Cc: mesa-stable
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20748>
(cherry picked from commit 0f5c8c3dc35561f23a5383cb05075898942ce0c5)