i965: Create backend_visitor fields for debugging messages.

i965: Create backend_visitor fields for debugging messages.

We introduce three new fields in backend_visitor:
- debug_enabled: whether or not INTEL_DEBUG & DEBUG_<stage flag>
- stage_name: "vertex", "fragment", etc. for use in messages
- stage_abbrev: "VS", "FS", etc. for use in messages

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>