drm/doc: move struct drm_crtc to in-line comments

And clean them up a bit, as usual.

v2: Fix nits (Sean).

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180709084016.23750-12-daniel.vetter@ffwll.ch