drm/i915: Remove redundant initialisation of per-ring IRQ waitqueues

The waitqueues are already initialised during ring initialisation so
kill the redundant and duplicated code to do so in each generations IRQ
installer.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>