glsl: Add a method to tell whether a built-in is available.

glsl: Add a method to tell whether a built-in is available.

We can simply call the stored predicate function.  If state is NULL,
just report that the function is available.

v2: Add a comment (requested by Paul Berry).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>