drm/i915/bdw: Provide a gen8 version of SRM

GEN8 now has a qword to code for 48bit addresses.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>