drm/i915: don't take runtime PM reference around freeze/thaw

We should be taking the right power well refs these days, so this
shouldn't be necessary.  It also gets in the way of re-using these
routines for S0iX states, as those need all the power saving features
enabled.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>