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>
i965: Remove redundant (and uninitialized) field vec4_generator::ctx.

We never noticed that this field was uninitialized because it is only
used in an error path that reports internal Mesa errors.

But it's silly to have it around anyway because &brw->ctx is
equivalent.

Should fix Coverity defect CID 1063351: Uninitialized pointer field
(UNINIT_CTOR) /src/mesa/drivers/dri/i965/brw_vec4_emit.cpp: 148

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>