drm/irq: kerneldoc polish

- Integrate into the drm DocBook
- Disable kerneldoc for functions not exported to drivers.
- Properly document the new drm_vblank_on|off and add cautious
  comments explaining when drm_vblank_pre|post_modesets shouldn't be
  used.
- General polish and OCD.

v2: Polish as suggested by Thierry.

Cc: Thierry Reding <thierry.reding@gmail.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>