fbcon: untangle fbcon_exit

There's a bunch of confusions going on here:
- The deferred fbcon setup notifier should only be cleaned up from
  fb_console_exit(), to be symmetric with fb_console_init()
- We also need to make sure we don't race with the work, which means
  temporarily dropping the console lock (or we can deadlock)
- That also means no point in clearing deferred_takeover, we are
  unloading everything anyway.
- Finally rename fbcon_exit to fbcon_release_all and move it, since
  that's what's it doing when being called from consw->con_deinit
  through fbcon_deinit.

To answer a question from Sam just quoting my own reply:

> We loose the call to fbcon_release_all() here [in fb_console_exit()].
> We have part of the old fbcon_exit() above, but miss the release parts.

Ah yes that's the entire point of this change. The release_all in the
fbcon exit path was only needed when fbcon was a separate module
indepedent from core fb.ko. Which means it was possible to unload fbcon
while having fbdev drivers registered.

But since we've merged them that has become impossible, so by the time the
fb.ko module can be unloaded, there's guaranteed to be no fbdev drivers
left. And hence removing them is pointless.

v2: Explain the why better (Sam)

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Claudio Suarez <cssk@net-c.es>
Cc: Du Cheng <ducheng2@gmail.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Link: https://patchwork.freedesktop.org/patch/msgid/20220405210335.3434130-17-daniel.vetter@ffwll.ch