drm/i915/bdw: Expand FADD to 64bit

For error state, like the recent modification to ACTHD, FADD also gets
an upper dword. This is useful for debug to make sure the fetch address
and head are similar.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>