drm: Fixup flip-work kerneldoc

Describe the fifo parameter. It seems like kerneldoc doesn't properly
handle fields defined using a macro, so it will end up complaining about
this anyway and not generate the documentation for it either. At least
the kerneldoc is now complete.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>