drm/i915: use drm_modeset_lock_all

Two exceptions:
- debugfs files only read information which is not related to crtc, so
  can stay on the modeset_config lock.
- Same holds for the edp vdd work in intel_dp.c. Add a corresponding
  WARN_ON and a comment next to the intel_dp struct fields for
  documentation.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>