drm/docs: Small cleanup in drm-uapi.rst

- Remove the outdated hunk about driver documentation which somehow
  got misplaced here in the split-up.

- Collect all the testing&validation stuff together and give the CRC
  section a heading for prettier output.

Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1483044517-5770-1-git-send-email-daniel.vetter@ffwll.ch