drm/todo: Update drm_fb_helper tasks

drm_fb_helper tasks are completed now hence remove them from
todo list.

Changes since v1:
* remove entire drm_fb_helper tasks from todo list. Daniel's
  "64914da2 drm/fbdev-helper: don't force restores" already fixes
  first one (Daniel)

Signed-off-by: Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200305120434.111091-8-pankaj.laxminarayan.bharadiya@intel.com