turnip: fix tu6_pack_border_color for z24
The value should be at the bottom 24 bits, not at the top.
dEQP-VK.pipeline.sampler.*
still passes. This fixes most of
dEQP-GLES31.functional.texture_border_clamp.formats.*depth*
on angle.
There are still failures regarding Z24S8 and stencil border color. angle uses channel 0 yet the hw expects channel 3. I need to check whether it is a angle bug or a turnip bug.