drm/i915: unify FLIP_DONE macro names

s/FLIPDONE/FLIP_DONE/ to make all FLIP_DONE macro names consistent.

No functional change.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>