gallium/ntt: Add support for PIPE_CAP_LOAD_CONSTBUF.
We needed to do this anyway to finish enabling NTT in general, but more
importantly: when we enabled sending NIR to the draw module, that broke
PIPE_CAP_LOAD_CONSTBUF drivers in the select/feedback paths if LLVM was
disabled.
Fixes:
44b7e1497f91 ("st/mesa: don't generate TGSI for the draw VS because it now supports NIR too")
(along with the rest of this MR)
Closes: #3996
Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8196>