drm_atomic_helper_setup_commit() auto-completes commit->flip_done when
state->legacy_cursor_update is true, but we know for sure that we want
a sync update when we call drm_atomic_helper_setup_commit() from
vc4_atomic_commit().

Explicitly set state->legacy_cursor_update to false to prevent this
auto-completion.

Fixes: 184d3cf4 ("drm/vc4: Use wait_for_flip_done() instead of wait_for_vblanks()")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181115105852.9844-2-boris.brezillon@bootlin.com