drm/i915/execlists: Check for ce->state before destroy

As we may cancel the ce->state allocation during context pinning (but
crucially after we mark ce as operational), that means we may be asked
to destroy a nonexistent ce->state. Given the choice in handing a
complex error path on pinning, and just ignoring the lack of state in
destroy, choice the latter for simplicity.

Reported-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180625100604.22598-1-chris@chris-wilson.co.uk