drm: Nerf drm_global_mutex BKL for good drivers

This catches the majority of drivers (unfortunately not if we take
users into account, because all the big drivers have at least a
lastclose hook).

With the prep patches out of the way all drm state is fully protected
and either prevents or can deal with the races from dropping the BKL
around open/close. The only thing left to audit are the various driver
hooks - by keeping the BKL around if any of them are set we have a
very simple cop-out!

Note that one of the biggest prep pieces to get here was making
dev->open_count atomic, which was done in

commit 7e13ad89
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Jan 24 13:01:07 2020 +0000

    drm: Avoid drm_global_mutex for simple inc/dec of dev->open_count

v2:
- Rebase and fix locking in drm_open() (Chris)
- Indentation fix in drm_release
- Typo fix in the commit message (Sam)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200204150146.2006481-6-daniel.vetter@ffwll.ch