winsys-glx: Reenable swap_region for llvmpipe and swrast

The bug that prevented MESA_copy_sub_buffer to work for swrast /
llvmpipe got fixed in mesa 10.1 git so enable it for mesa 10.1+.

https://bugzilla.gnome.org/show_bug.cgi?id=721450

When landing the patch, it was tweaked to #include "cogl-version.h" to
avoid a compiler warning about COGL_VERSION_ENCODE being implicitly
defined. -- Robert Bragg

Reviewed-by: Robert Bragg <robert@linux.intel.com>
winsys-glx: Fix swap region to remain disabled for old mesa

Commit a750f80c6aaa was supposed to enable it for newer mesa but was wrong,
fix that.