drm/i915: add comments on what stage a given PM handler is called

This will hopefully make it easier to navigate the code without the need
to consult the full PM documentation.

v2:
- add a comment that the freeze handler is also called after rebooting
- add a comment that the thaw handler is also called to recover from
  errors (Ville)
- add the PM event names (PMSG_THAW etc.) for reference (Ville)
- add comments that s0ix can be handled both via system and runtime
  suspend (Ville)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>