radeonsi: fix polygon offset scale
The value was copied from r300g, which uses 1/12 subpixels, but this hw uses 1/16 subpixels. Fixes piglit: gl-1.4-polygon-offset (formerly a glean test) Reviewed-by:Michel Dänzer <michel.daenzer@amd.com> Cc: mesa-stable@lists.freedesktop.org (cherry picked from commit bfac8ba9)