drm/fb-helper: Remove drm_fb_helper_defio_init() and update docs

There are no users of drm_fb_helper_defio_init(), so we can remove
it. The documentation around defio support is a bit misleading and
should mention compatibility issues with SHMEM helpers. Clarify this.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191025092759.13069-2-tzimmermann@suse.de