gallium/dri3: Drop loader_dri3_buffer::size

It's never set outside of the automatic zero from calloc().  Having it
in there is decpetive.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36014>
2 files changed