drm/i915: Enable PM Interrupts target via Display Interface.

In BDW, Apart from unmasking up/down threshold interrupts. we need
to umask bit 32 of PM_INTRMASK to route interrupts to target via Display
Interface.

v2: Add (1<<31) mask (Ville)

v3: Add Gen check for the mask (ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>