drm/i915: Fix typo in kerneldoc function name

A forgetful copy'n'paste left the name of the old function intact, and
did not introduce the new function 'i915_request_is_ready'

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200117101639.2908469-1-chris@chris-wilson.co.uk