r600g: always map uninitialized buffer range as unsynchronized

r600g: always map uninitialized buffer range as unsynchronized

Any driver can implement this simple and efficient optimization.
Team Fortress 2 hits it always. The DISCARD_RANGE codepath is not even used
with TF2 anymore, so we avoid a ton of useless buffer copies.

Tested-by: Andreas Boll <andreas.boll.dev@gmail.com>
NOTE: This is a candidate for the 9.1 branch.