nv30: avoid setting user_priv without setting cur_ctx

nv30: avoid setting user_priv without setting cur_ctx

The whole user_priv thing is a mess, but as long as it's there, it
basically has to map 1:1 to the cur_ctx. Unfortunately we were setting
user_priv to some context, then that context could get deleted without
any draws/validations in it, leading user_priv to become NULL, with
cur_ctx still pointing at some old context. Then we wouldn't run the
switch logic, which in turn led to a NULL bufctx being dereferenced.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102349
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>