- Move all the pm helpers into igt_pm.c. No idea why that wasn't done
  in the original commit that created igt_pm.c.
- Add missing docs where needed.

Cc: David Weinehall <david.weinehall@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>