gallium/pipebuffer: fix parenthesis location

Without this the return value will never get set to -1. This
was first added in 49866c8f and copied in 2b396eee.

Fixes: 2b396eee "gallium/pb_cache: add a copy of cache bufmgr independent of pb_manager"

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102342
(cherry picked from commit 7e9b7ec0)