Lots of added text here since I think the various control flow bits
are worth explaining a bit better.

v2: Fix conflict with Boris' no_vblank addition.
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180709084016.23750-15-daniel.vetter@ffwll.ch