drm/i915/gt: Hook up intel_context_fini()

Prior to freeing the struct, call the fini function to cleanup the
common members. Currently this only calls the debug functions to mark
the structs as destroyed, but may be extended to real work in future.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190718070024.21781-2-chris@chris-wilson.co.uk