nvc0: don't put text segment into bufctx

nvc0: don't put text segment into bufctx

The text segment is shared among multiple contexts, while each one has
its own bufctx. So when reallocating the text segment, some contexts may
end up with stale values in their bufctx's. Instead limit the exposure
to the bufctx to within a single draw.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>