nouveau/codegen: Use a NULL format for PIPE_FORMAT_NONE for images

nouveau/codegen: Use a NULL format for PIPE_FORMAT_NONE for images

Get rid of our own FMT_NONE and just use a NULL format in that case.
We're already checking su->tex.format != NULL in a bunch of cases but
none of those NULL checks are doing anything right now execpt detecting
when its an actual texture op and not an image op.  We'd like them to
start also applying for image load/store with an unspecified format.

Reviewed-by: M Henning <drawoc@darkrefraction.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24327>