mesa: call _mesa_make_extension_string only as needed

mesa: call _mesa_make_extension_string only as needed

As of previous commit we removed the extension overrides from this
function.

Thus we no longer need to call it during MakeCurrent, so we can
construct the extensions string when needed - _mesa_GetString.

This commit effectively reverts a879d14ecf8 ("mesa: initialize extension
string when context is first bound")

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Brian Paul <brianp@vmware.com>