drm/i915: ValleyView IRQ support

ValleyView has a new interrupt architecture; best to put it in a new set
of functions.  Also make sure the ring mask functions handle ValleyView.

FIXME: fix flipping; need to enable interrupts and call prepare/finish

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>