drm/doc: Polish irq helper documentation

Pull a (much shorter) overview into drm_irq.c, and instead put the
callback documentation into in-line comments in drm_drv.h.

v2: Move the include stanzas back to the split-up patch (Stefan).

Cc: Stefan Agner <stefan@agner.ch>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170531092253.12833-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_vblank.c

@@ -363,6 +363,9 @@ static void vblank_disable_fn(unsigned long arg)
  * @dev: DRM device
  *
  * This function cleans up any resources allocated in drm_vblank_init.
+ *
+ * Drivers which don't use drm_irq_install() need to set &drm_device.irq_enabled
+ * themselves, to signal to the DRM core that vblank interrupts are enabled.
  */
 void drm_vblank_cleanup(struct drm_device *dev)
 {