drm/i915: support page flipping on ValleyView

And restructure the IRQ handling a little.  We can use pipestat for most
things, and make sure we don't affect pipe events when enabling and
disabling vblank interupts.

We can leave vblank interrupts masked but enabled so we're not dependent
on the first client to toggle the disable timer.  We can also mask all
render based interrupts, since the ring code will handle unmasking them
for us.

v2: roll in vblank masking, remove unneeded variable (Daniel)

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