drm/i915: cleanup rc6 code

Cleanup several aspects of the rc6 code:
  - misnamed intel_disable_clock_gating function (was only about rc6)
  - remove commented call to intel_disable_clock_gating
  - rc6 enabling code belongs in its own function (allows us to move the
    actual clock gating enable call back into restore_state)
  - allocate power & render contexts up front, only free on unload
    (avoids ugly lazy init at rc6 enable time)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[ickle: checkpatch cleanup]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>