drm/nouveau/imem/nv50: move slow-path locking into rd/wr functions
This is to simplify upcoming changes. The slow-path is something that
currently occurs during bootstrap of the BAR2 VMM, while backing up an
object during suspend/resume, or when BAR2 address space runs out.
The latter is a real problem that can happen at runtime, and occurs in
Fedora 26 already (due to some change that causes a lot of channels to
be created at login), so ideally we'd prefer not to make it any slower.
We'd also like suspend/resume speed to not suffer.
Upcoming commits will solve those problems in a better way, making the
extra overhead of moving the locking here a non-issue.