drm/i915/bxt: Enable DC5 during runtime resume

Right after runtime resume we know that we can re-enable DC5, since we
just disabled DC9 and power well 2 is disabled. So enable DC5 explicitly
instead of delaying this until the next time we disable power well 2.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461173277-16090-5-git-send-email-imre.deak@intel.com