drm/doc: Polish docs for drm_property&drm_property_blob

- remove kerneldoc for drm-internal functions
- drm_property_replace_global_blob isn't actually atomic, and doesn't
  need to be. Update docs&comments to match
- document all the types and try to link things a bit better
- nits all over

v2: Appease checkpatch in the moved code (Archit)

Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160829082757.17913-9-daniel.vetter@ffwll.ch