i965: Use atomic ops in get_new_program_id().

i965: Use atomic ops in get_new_program_id().

We have a nice utility function for this, which eliminates the need for
locking stuff.  This isn't really performance critical, but it's less
code to use the atomic.

p_atomic_inc_return does pre-increment rather than post-increment, so we
change screen->program_id to be initialized to 0 instead of 1.  At which
point, we can just delete the initialization because intel_screen is
rzalloc'd.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>