drm/i915: abstract away ring-specific irq_get/put

Inspired by Ben Widawsky's patch for gen6+. Now after restructuring
how we set up the ring vtables and parameters, we can do this right.

This kills the bsd specific get/put_irq functions, they're now the
same.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>