drm/i915: s/__i915_wait_request/i915_wait_request/

There is only one wait on request function now, so drop the "expert"
indication of leading __.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470293567-10811-21-git-send-email-chris@chris-wilson.co.uk