drm/i915/execlists: Refactor out complete_preempt_context()

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 <[email protected]>
Cc: Jeff McGee <[email protected]>
Cc: Michał Winiarski <[email protected]>
Reviewed-by: Jeff McGee <[email protected]> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]