drm/i915/execlists: Protect peeking at execlists->active

Now that we dropped the engine->active.lock serialisation from around
process_csb(), direct submission can run concurrently to the interrupt
handler. As such execlists->active may be advanced as we dequeue,
dropping the reference to the request. We need to employ our RCU request
protection to ensure that the request is not freed too early.

Fixes: df403069 ("drm/i915/execlists: Lift process_csb() out of the irq-off spinlock")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191009100955.21477-1-chris@chris-wilson.co.uk