drm/todo: Add tinydrm refactoring ideas

Discussed with Noralf on the list a bit.

An open question is tinydrm vs. drm_panel, but until we have a clear
idea what's really needed in that space, I think it's best to just
move forward with what we have.

Cc: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170322083617.13361-10-daniel.vetter@ffwll.ch