drm/i915/execlists: Assert that we don't get mixed IDLE_ACTIVE | COMPLETE events

If IDLE_ACTIVE is set, then all other bits are invalid. For us, we can
assert that if we see a COMPLETE | PREEMPTED event, then it should be
impossible for it to also contain an IDLE_ACTIVE flag.

Suggested-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
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>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171120123458.23242-3-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/intel_lrc.c

@@ -874,6 +874,9 @@ static void execlists_submission_tasklet(unsigned long data)
 			if (!(status & GEN8_CTX_STATUS_COMPLETED_MASK))
 				continue;
 
+			/* We should never get a COMPLETED | IDLE_ACTIVE! */
+			GEM_BUG_ON(status & GEN8_CTX_STATUS_IDLE_ACTIVE);
+
 			if (status & GEN8_CTX_STATUS_COMPLETE &&
 			    buf[2*head + 1] == PREEMPT_ID) {
 				execlists_cancel_port_requests(execlists);