drm: Add TODO item for fbdev driver conversion

The DRM TODO list now contains an entry for converting fbdev
drivers over to DRM.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191017074705.9140-2-tzimmermann@suse.de