Call softpipe_unmap_surfaces() in softpipe_flush().

This fixes a DRM BO failure upon swapbuffers caused by the color buffer
still being mapped.
This is a bit heavy handed since we don't always need to unmap buffers
when flushing.  Need to pass a flag to flush() or design a new function.

src/mesa/pipe/softpipe/sp_flush.c

@@ -63,5 +63,13 @@ softpipe_flush( struct pipe_context *pipe,
 
    if (softpipe->sbuf_cache)
       sp_flush_tile_cache(softpipe->sbuf_cache);
+
+   /* Need this call for hardware buffers before swapbuffers.
+    *
+    * there should probably be another/different flush-type function
+    * that's called before swapbuffers because we don't always want
+    * to unmap surfaces when flushing.
+    */
+   softpipe_unmap_surfaces(softpipe);
 }