drm/i915: Only reset seqno if actually idle

Before we can reset the seqno, we have to be sure the engines are idle.
In debugfs/i915_drop_caches_set, we do wait_for_idle but allow ourselves
to be interrupted. We should only proceed to reset the seqno then if we
were not interrupted, and so also avoid overwriting the error status.

References: https://bugs.freedesktop.org/show_bug.cgi?id=108133
Fixes: 6b048706 ("drm/i915: Forcibly flush unwanted requests in drop-caches")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004082119.24970-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_debugfs.c

@@ -4205,7 +4205,7 @@ i915_drop_caches_set(void *data, u64 val)
 						     I915_WAIT_LOCKED,
 						     MAX_SCHEDULE_TIMEOUT);
 
-		if (val & DROP_RESET_SEQNO) {
+		if (ret == 0 && val & DROP_RESET_SEQNO) {
 			intel_runtime_pm_get(i915);
 			ret = i915_gem_set_global_seqno(&i915->drm, 1);
 			intel_runtime_pm_put(i915);