drm/i915/execlists: Refactor out complete_preempt_context()

As a complement to inject_preempt_context(), follow up with the function
to handle its completion. This will be useful should we wish to extend
the duties of the preempt-context for execlists.

v2: And do the same for the guc.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jeff McGee <jeff.mcgee@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Jeff McGee <jeff.mcgee@intel.com> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20180516183355.10553-3-chris@chris-wilson.co.uk