drm/fb-helper: Remove mention of CONFIG_FB_DEFERRED_IO in docs

This was forgotten to fixup in the latest version of the deferred_io
patch which made FB_DEFERRED_IO mandatory.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1462982962-10530-2-git-send-email-noralf@tronnes.org