drm/i915: Skip context_barrier emission for unused contexts

The intent was to skip unused HW contexts by checking ce->state.
However, this only works for execlists where the ppGTT pointers is
stored inside the HW context. For gen7, the ppGTT is alongside the
logical state and must be updated on all active engines but, crucially,
only on active engines. As we need different checks, and to keep
context_barrier_task() agnostic, pass in the predicate.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110836
Fixes: 62c8e423

 ("drm/i915: Skip unused contexts for context_barrier_task()")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190604152408.24468-1-chris@chris-wilson.co.uk