i965: Move get_hw_prim_for_gl_prim to brw_util.c

i965: Move get_hw_prim_for_gl_prim to brw_util.c

It's used by brw_compile_gs in brw_vec4_gs_visitor.cpp so it needs to be in
a file that's linked into libi965_compiler.la.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>