drm/i915: Fix to Enable GT/PM Interrupts

Programing GT IER interrupts was fumbled while enabling Interrupts for
gen8

We forgot to program PM IER interrupt in gen8_gt_irq_postinstall based
on the new  re-worked interrupt routines.

v2: Kill the loop and init GT interrupts individually (Ville)

Signed-off-by: Deepak S <deepak.s@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Adjust commit message as per discussion with Deepak.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>