drm/todo: Convert drivers to generic fbdev emulation

This replaces the original TODO item for drm_fb_helper_fbdev_setup()
and _teardown(), which are deprecated.

v2:
	* remove driver-specific comments
	* list some basic requirements
	* keep a TODO item on drm_fb_helper_init()

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191106124727.11641-2-tzimmermann@suse.de