i965: Emit a performance warning on conditional rendering.

We have a CPU-side implementation of conditional rendering; it really
should be done on the GPU.  It's not necessarily that hard, but nobody
has gotten to fixing it yet.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>

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

@@ -539,6 +539,11 @@ void brw_draw_prims( struct gl_context *ctx,
 
    assert(unused_tfb_object == NULL);
 
+   if (ctx->Query.CondRenderQuery) {
+      perf_debug("Conditional rendering is implemented in software and may "
+                 "stall.  This should be fixed in the driver.\n");
+   }
+
    if (!_mesa_check_conditional_render(ctx))
       return;