svga: Update the backing resource only if needed

svga: Update the backing resource only if needed

This patch adds a timestamp in svga_surface structure to keep track
of when the backing surface is last sync with the original resource.
This helps to avoid unnecessary surface copy from the original
resource to the backing surface if the original resource has not
since been modified.

This reduces the amount of surface copy with CinebenchR15.

Tested with CinebenchR15, mtt glretrace.

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