zink: fix emulation of no mipfilter

zink: fix emulation of no mipfilter

This approach is taken from the Vulkan spec[1], where a robust maxLod
of 0.25 is proposed instead of 0.0.

This has the effect of allowing room for both minification and
magnification filters, yet still rounding down to the right miplevel in
the end.

[1]: https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/VkSamplerCreateInfo.html#_description

Fixes: 8d46e35d16e ("zink: introduce opengl over vulkan")
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9649>