drm/i915/ringbuffer: Move all default irq vfuncs init to a separate func

Just plonk all the default irq vfuncs together in one function to keep
the initialisers of reasonable size.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467361093-20209-2-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>