drm: Polish fbdev helper struct docs

Mostly this is just adding extensive docs for the callbacks, but also
a few other additions.

v2: Use FIXME comments to annotate helper hooks that should be
replaced.

v3: Small nits (Thierry).

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-2-git-send-email-daniel.vetter@ffwll.ch


Reviewed-by: Thierry Reding <treding@nvidia.com>