drm: some KMS todo ideas

Inspired by discussions with Keith and Ville.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Keith Packard <keithp@keithp.com>
Acked-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171017162918.8380-1-daniel.vetter@ffwll.ch