intel: Add missing perf debug for a stall on mapping a BO.

I was testing the ARB_debug_output code and wrote an obvious sample that
should have hit this, and got confused that my ARB_debug_output was
broken.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

src/mesa/drivers/dri/intel/intel_buffer_objects.c

@@ -330,6 +330,8 @@ intel_bufferobj_map_range(struct gl_context * ctx,
 	    drm_intel_bo_unreference(intel_obj->buffer);
 	    intel_bufferobj_alloc_buffer(intel, intel_obj);
 	 } else {
+            perf_debug("Stalling on the GPU for mapping a busy buffer "
+                       "object\n");
 	    intel_flush(ctx);
 	 }
       } else if (drm_intel_bo_busy(intel_obj->buffer) &&