st: add ST_FLUSH_WAIT to st_context_flush()

When st_context_flush() is called with ST_FLUSH_WAIT,
the function will return after the fence is completed.

Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/include/state_tracker/st_api.h

@@ -160,6 +160,7 @@ enum st_context_resource_type {
  */
 #define ST_FLUSH_FRONT                    (1 << 0)
 #define ST_FLUSH_END_OF_FRAME             (1 << 1)
+#define ST_FLUSH_WAIT                     (1 << 2)
 
 /**
  * Value to st_manager->get_param function.