zink: scale depth bias by factor of 2

Merged Mike Blumenkrantz requested to merge zmike/mesa:zink-units into main

this is enough to fix CTS

affects/fixes: dEQP-GLES3.functional.polygon_offset.default_render_with_units dEQP-GLES3.functional.polygon_offset.fixed16_render_with_units dEQP-GLES3.functional.polygon_offset.fixed24_render_with_units

Merge request reports