drm/i915: kill cargo-culted locking from power well code

We may not concurrently change the power wells code. Which
is already guaranteed since modesets aren't concurrent. That
leaves races against setup/teardown/suspend/resume, and for
those we already (try) rather hard not to hit concurrent
modesets.

No debug WARN_ON added since that would require us to grab the
modeset locks in init/suspend code. Which is again just cargo
culting since just grabbing the locks in those paths isn't good
enough, we need the right order of operations, too.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>