etnaviv: fix depth bias

Fixes remaining failures in these deqp tests (tested on GC3000/GC7000L):
dEQP-GLES2.functional.polygon_offset.*

Fixes: 6c3c05dc ("etnaviv: fix polygon offset")

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
(cherry picked from commit 7b524e1a)