dzn: Handle depth bias for point fill mode emulation

dzn: Handle depth bias for point fill mode emulation

Fixes dEQP-VK.draw.renderpass.depth_bias.depth_bias_triangle_list_point

This is not complete, there's no slope scale or clamp handling, but it
does handle static or dynamic (though dynamic is untested).

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22639>