drm/i915: Make __wait_seqno non-static and rename to __i915_wait_seqno

So that it can be used by the flip code to wait for rendering without
holding any locks.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>