drm/fb-cma-helper: Hook up to DocBook and fix some docs

Hook up fb_cma_helper to DocBook. Remove mention of
CONFIG_FB_DEFERRED_IO in the docs, which was forgotten in the latest
version of the deferred_io patch.
Use & when referencing drm_mode_config_funcs in docs.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1462982962-10530-3-git-send-email-noralf@tronnes.org