drm/i915/gen8+: Do not enable DPF interrupt since the handler does not exist

Looks like DPF was not implemented for gen8+ but the IER and IMR
are still enabled on initialization.

Since there is no code to handle this interrupt, gate the irq
enablement behind HAS_L3_DPF in case the feature gets enabled
in the future.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>