r600g: fix polygon offset scale
The value was copied from r300g, which uses 1/12 subpixels, but this hw
uses 1/16 subpixels.
Should fix piglit: gl-1.4-polygon-offset (formerly a glean test)
(untested, ported from radeonsi)
Reviewed-by: Edward O'Callaghan <eocallaghan at alterapraxis.com>
Reviewed-by:
Alex Deucher <alexander.deucher@amd.com>
Cc: mesa-stable@lists.freedesktop.org