drm/gem: Update/Polish docs

A bunch of things have been removed meanwhile and docs not fully
brought up to speed. And a few gaps closed where I noticed missing
kerneldoc while reading through the overview sections.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1445533889-7661-3-git-send-email-daniel.vetter@ffwll.ch


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