drm: Improve integration with syscons. Move taskqueue handling to syscons.

drm: Improve integration with syscons. Move taskqueue handling to syscons.

* Adds asynctd_lk to syscons' softc to synchronize unregister_framebuffer
  code with the asynchronous screen refresh threads.

* Use a generation counter in syscons to check if sc_update_render() needs
  to update the VT resolution/mode. Remove struct fb_info *fbi pointer
  from scr_stat, and always check the struct fb_info *fbi pointer in the
  softc struct instead.

* Use videio_in_progress flag to make initial fb_set_par call a bit safer.

* Moves driver callbacks from struct fb_info itself into struct fb_ops,
  and adds fb_blank and fb_debug_enter callbacks which will be wired up
  in future commits.

* Add unregister_framebuffer() function to syscons, which is needed for
  cleanly detaching a drm graphics driver which is used as framebuffer
  console. The screen will turn off, blank or display a frozen screen,
  after the graphics driver has detached.

* Use drm_fb_helper_unregister_framebuffer() and drm_fb_helper_release_fbi()
  functions in radeon and i915, which are needed for cleanly detaching the
  drivers.

* With radeonkms detaching and re-attaching is working very reliably
  on an AMD G-T44R APU (i.e. HD6250 graphics).

* Detaching i915 on a Haswell notebook barely worked with several errors,
  and trying to re-attach i915 caused the machine to hang.