drm: Document drm_atomic_*_get_property

Yes these are internal functions and not exported and we generally
don't document them. But for symmetry with the _set_property functions
(which are exported for the atomic helpers) I'd like to document them.
Upcoming vtable kerneldoc will reference both the set and get_property
functions.

v2: Suggestions from Thierry.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-17-git-send-email-daniel.vetter@ffwll.ch