drm/i915: update the PC8 and runtime PM documentation

Now that PC8 got much simpler, there are less things to document.
Also, runtime PM already has a nice documentation, so we don't need to
re-explain it on our driver.

v2: - Rebase.
    - Fix typo (Jesse).

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>