llvmpipe: multiply polygon offset units by 2

I was trying to fix this test, but noticed brw_clip.c in i965 does a * 2 here, and it seems to fix this test as well.

Fixes: dEQP-GLES2.functional.polygon_offset.default_displacement_with_units

Reviewed-by: Roland Scheidegger sroland@vmware.com

Split out from !5974