drm/i915: drop DPF code for gen8+

The only gen8+ platform that has the feature is BDW, but we don't define
the feature flag on any BDW platform and we only have partial support in
the gen8 path (irq enabling code, but no handler).
The only thing we could do in the irq handler is report the error
to userspace, but no one asked/cared about that since BDW was
released so it is relatively safe to assume that even if we added the
message no one would look at it. Just drop the dead code from the
driver instead.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190109213147.16851-1-daniele.ceraolospurio@intel.com