i965: Use safer pointer arithmetic in gather_oa_results()

This patch reduces the likelihood of pointer arithmetic overflow bugs in
gather_oa_results(), like the one fixed by b69c7c5dac.

I haven't yet encountered any overflow bugs in the wild along this
patch's codepath. But I get nervous when I see code patterns like this:

   (void*) + (int) * (int)

I smell 32-bit overflow all over this code.

This patch retypes 'snapshot_size' to 'ptrdiff_t', which should fix any
potential overflow.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>

src/mesa/drivers/dri/i965/brw_performance_monitor.c

@@ -907,7 +907,7 @@ gather_oa_results(struct brw_context *brw,
       return;
    }
 
-   const int snapshot_size = brw->perfmon.entries_per_oa_snapshot;
+   const ptrdiff_t snapshot_size = brw->perfmon.entries_per_oa_snapshot;
 
    /* First, add the contributions from the "head" interval:
     * (snapshot taken at BeginPerfMonitor time,