drm/i915: Rework sdvo proxy i2c locking

lockdep complaints about a locking recursion for the i2c bus lock
because both the sdvo ddc proxy bus and the gmbus nested within use
the same locking class. It's not really a deadlock since we never nest
the other way round, but it's annoying.

Fix it by pulling the gmbus locking into the i2c lock_ops for both
i2c_adapater and making sure that the ddc_proxy_xfer function is
entirely lockless.

Re-layouting the extracted function resulted in some whitespace
cleanups, I figured we might as well keep them.

v2: Review from Chris:
- s/locked/unlocked/ since I got the naming backwards
- Use the vfuncs of the proxied adatper instead of re-rolling copies.
  That's more consistent with the other proxying we're doing.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170726132647.31833-1-daniel.vetter@ffwll.ch