svga: minimize surface flush

svga: minimize surface flush

Currently, we always do a surface flush when we try to establish
a synchronized write transfer map. But if the subresource has not
been modified, we can skip the surface flush. In other words,
we only need to do a surface flush if the to-be-mapped subresource
has been modified in this command buffer.

With this patch, lightsmark2008 shows about 15% performance improvement.

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