gallium: Add a cap for offset_units_unscaled

gallium: Add a cap for offset_units_unscaled

D3D9 has a different behaviour for depth bias.

For OGL/D3D1X, the depth bias unit is the
minimal resolvable value for the depth buffer,
which depends on the format (and has different
behaviour for float depth buffers).

For D3D9, the depth bias unit is 1.0f.

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>