drm: Fixup failure paths in drm_atomic_helper_set_config

I've screwed this up when removing the legacy backoff hack.

Fixes: 38b6441e ("drm/atomic-helper: Remove the backoff hack from set_config")
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170329174136.10330-1-daniel.vetter@ffwll.ch