i915g: Don't avoid flushing when we have a pending fence.

Otherwise the fence will never arrive.
Also check for a NULL i915->batch.

NOTE: This is a candidate for the 8.0 branch.

src/gallium/drivers/i915/i915_flush.c

@@ -45,7 +45,10 @@ static void i915_flush_pipe( struct pipe_context *pipe,
 
    draw_flush(i915->draw);
 
-   if (i915->batch->map == i915->batch->ptr) {
+   /* Only shortcut this if we have no fence, otherwise we must flush the
+    * empty batchbuffer to get our fence back.
+    */
+   if (!fence && i915->batch && (i915->batch->map == i915->batch->ptr)) {
       return;
    }