The brw_disasm_info header is included by certain tools in order to get
shader assembly from binaries so it's a semi-external header.  Including
brw_cfg.h also pulls in brw_shader.h so you end up getting quite a bit
of our back-end compiler internals.  Instead, make the couple of forward
declarations we need and make the header more stand-alone.  This fixes
the meson build.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Fixes: 4f82b172