drm/i915: add runtime PM support on Haswell

Now that all the infrastructure is in place and all the tests from
pm_pc8 pass, we can finally enable the feature.

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