drm/doc: Include drm_of.c helpers

Fixes a dead link I spotted in the struct drm_crtc docs. Comments
themselves are in a surprisingly good state.

v2: Fix subject typo (Sean).

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180709084016.23750-14-daniel.vetter@ffwll.ch