drm/i915: Only skip requests once a context is banned

If we skip before banning, we have an inconsistent interface between
execbuf still queueing valid request but those requests already queued
being cancelled. If we only cancel the pending requests once we stop
accepting new requests, the user interface is more consistent.

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Fixes: 821ed7df ("drm/i915: Update reset path to fix incomplete requests")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: <stable@vger.kernel.org> # v4.9+
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170105170059.344-1-chris@chris-wilson.co.uk