drm/doc: Add TODO list

This commit adds a TODO list to the GPU driver developer's guide. The
content was taken from the DRMJanitors page on the X.Org wiki:

	https://www.x.org/wiki/DRMJanitors/



The goal is to track a list of refactorings that would be nice to see
merged eventually. Sometimes these would be encountered during patch
review on the mailing list, and at other times one can come across
these while working in a specific area of code.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170207175113.2793-1-thierry.reding@gmail.com