glsl: Rename the fourth argument to get_interface_instance.

glsl: Rename the fourth argument to get_interface_instance.

Interface declarations have two names associated with them: the block
name and the instance name.  It's the block name that needs to be
passed to get_interface_instance().  This patch renames the argument
so that there's no confusion.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>