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