mesa: Move per-pixel Z pack functions to swrast.

Only swrast uses them, and this makes it clearer why we only need partial
format support for them, avoids linker work deleting them for gallium
drivers, and means they will automatically go away when we get to finally
delete swrast.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10336>
4 files changed