Comment new vblank routines and fixup several issues:

  - use correct refcount variable in get/put routines
  - extract counter update from drm_vblank_get
  - make signal handling callback per-crtc
  - update interrupt handling logic, drivers should use drm_handle_vblank
  - move wakeup and counter update logic to new drm_handle_vblank routine
  - fixup usage of get/put in light of counter update extraction
  - fix longstanding bug in signal code, update pending counter only
    *after* we're sure we'll setup signal handling