i965/bufmgr: Skip wait ioctl when not busy.

If the buffer is idle, we I915_GEM_WAIT will return immediately,
so we may as well skip the ioctl altogether.  We can't trust the
"idle" flag for external buffers, but for most, it should be fine.

Reviewed-by: Matt Turner <mattst88@gmail.com>

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

@@ -924,6 +924,10 @@ brw_bo_wait(struct brw_bo *bo, int64_t timeout_ns)
    struct drm_i915_gem_wait wait;
    int ret;
 
+   /* If we know it's idle, don't bother with the kernel round trip */
+   if (bo->idle && !bo->external)
+      return 0;
+
    memclear(wait);
    wait.bo_handle = bo->gem_handle;
    wait.timeout_ns = timeout_ns;