drm/i915: get a runtime PM ref for debugfs entries where needed

These debugfs entries access registers that need the D0 power state so
get an RPM ref for them.

v2:
- for all these entries we only need D0 state, so get only an RPM ref,
  not a power domain ref (Daniel, Paulo)
- the dpio entry is not an issue any more as it got removed (Ville)
- restore commit message from v1 (Paulo)

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>