drm/i915: Extract context unpinning to its own function

Will enable cleaner implementation of a following fix and
easier code unification in the future.

Idea and code by Chris Wilson.

v2: Do not return before last_contexts on engines are unpinned.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>