r600g: Implement POLYGON_OFFSET_UNITS_UNSCALED

r600g: Implement POLYGON_OFFSET_UNITS_UNSCALED

Empirical tests show that the polygon offset
behaviour is entirely determined by the content of
the PA_SU_POLY_OFFSET states, and not by the depth buffer
format bound.

PA_SU_POLY_OFFSET seems to directly set the parameters of
the polygon offset formula, and setting 0 for
PA_SU_POLY_OFFSET_DB_FMT_CNTL (ie setting the unorm depth
bias behaviour with a scale of 2^0 = 1.0f) gives the unscaled
behaviour.

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>