r600g: Use a fake reloc to sleep for fences

r600g: Use a fake reloc to sleep for fences

r300g is able to sleep until a fence completes rather than busywait because
it creates a special buffer object and relocation that stays busy until the
CS containing the fence is finished.

Copy the idea into r600g, and use it to sleep if the user asked for an
infinite wait, falling back to busywaiting if the user provided a timeout.

Note: this is a candidate for the stable branches.

Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>