drm/i915: Remove unneeded drm_device pointer from intel_ring_init_seqno()

We only use drm_i915_private within the function, so delete the unneeded
drm_device local.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460010558-10705-3-git-send-email-chris@chris-wilson.co.uk