i965: Rename brw_bo::offset64 to gtt_offset.

i965: Rename brw_bo::offset64 to gtt_offset.

We can drop the meaningless "64" suffix - libdrm_intel originally had
an "offset" field that was an "unsigned long" which was the wrong size,
and we couldn't remove/alter that field without breaking ABI, so we had
to add a uint64_t "offset64" field.

"gtt_offset" is also more descriptive than "offset".

(Patch originally written by Ken, but Chris suggested a better name and
supplied the giant comment making up the bulk of the patch, so I changed
the authorship to him.)

Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>