zink: enable tgsi texcoord pipe cap

zink: enable tgsi texcoord pipe cap

this requires some modifications to the ntv slot remapping, as we definitely
don't want to reserve another 25% of the available slots for the (deprecated)
texcoord varyings

now we remap VARYING_SLOT_TEX[n] to the last available slots, which lets us avoid
needing to do permanent reservation, and we check to make sure that we haven't
seen the corresponding texcoord varying any time we emit a non-texcoord varying in
that slot

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5551>