drm/Documentation: Refine TODO for backlight helpers in tinydrm

Add a summary which resulted from discussions on what should
be done to refactor backlight helpers in tinydrm so that
they can be used in other drivers as well.

Signed-off-by: Meghana Madhyastha <meghana.madhyastha@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170927105116.GA30391@meghana-HP-Pavilion-Notebook