drm/doc: Update copyright

I've done quite a bit of cleanups, clarifications and mostly
integrating kerneldoc. So I guess I should add myself.

Also split up the copyright notices per holder to make it clear which
year ranges are covered.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>