intel: Add always_flush_cache driconf option for debugging cache flush failure.

I keep wanting to hack this knob in as a one-time thing, so it seemed useful
to have all the time.

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

@@ -71,10 +71,11 @@ PUBLIC const char __driConfigOptions[] =
    DRI_CONF_SECTION_END
    DRI_CONF_SECTION_DEBUG
      DRI_CONF_NO_RAST(false)
+     DRI_CONF_ALWAYS_FLUSH_CACHE(false)
    DRI_CONF_SECTION_END
 DRI_CONF_END;
 
-const GLuint __driNConfigOptions = 6;
+const GLuint __driNConfigOptions = 7;
 
 #ifdef USE_NEW_INTERFACE
 static PFNGLXCREATECONTEXTMODES create_context_modes = NULL;