drm/todo: Better defio support in the generic fbdev emulation

The current one essentially means you need CMA or a vmalloc backed
object, which makes fbdev emulation a special case.

Since implementing this will be quite a bit of work, capture the idea
in a TODO.

Cc: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190107102238.7789-1-daniel.vetter@ffwll.ch