drm/i915/bxt: Enable runtime PM

With the preceding fixes runtime PM should be functional, I could
runtime suspend/resume the device without problems.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: David Weinehall <david.weinehall@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459515767-29228-17-git-send-email-imre.deak@intel.com