drm/i915/skl: Enabling processing of Turbo interrupts

Earlier Turbo interrupts were not being processed for SKL,
as something was amiss in turbo programming for SKL.
Now missing changes have been added, so enabling the Turbo
interrupt processing for SKL.

Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>