drm/i915/execlists: Move insert_request()

Move insert_request() earlier to avoid a forward declaration in a later
patch.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170916204414.32762-2-chris@chris-wilson.co.uk