drm/todo: Clarify situation around fbdev and defio

The TODO item is misleading and makes it seem as if fbdev emulation
cannot be used with SHMEM. Rephrase the text to describe the current
situation more correctly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191025092759.13069-3-tzimmermann@suse.de