i965: Don't try to fall back when creating unrecognized program targets.

i965: Don't try to fall back when creating unrecognized program targets.

If brwNewProgram is asked to create a program for an unrecognized
target, don't bother falling back on _mesa_new_program().  That just
hides bugs.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
v2: Use assert() rather than _mesa_problem().

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>