drm: Fix kerneldoc for atomic_async_update

The enumeration of FIXMEs wasn't indented properly.

Fixes: fef9df8b ("drm/atomic: initial support for asynchronous plane update")
Cc: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170731111733.10507-1-daniel.vetter@ffwll.ch