drm: Push drm_global_mutex locking in drm_open

We want to only take the BKL on crap drivers, but to know whether
we have a crap driver we first need to look it up. Split this shuffle
out from the main BKL-disabling patch, for more clarity. Historical
aside: When the kernel-wide BKL was removed, it was replaced by
drm_global_mutex within the scope of the drm subsystem hence why these
two things are (almost) interchangeable as concepts here.

Since the minors are refcounted drm_minor_acquire is purely internal
and this does not have a driver visible effect.

v2: Push the locking even further into drm_open(), suggested by Chris.
This gives us more symmetry with drm_release(), and maybe a futuer
avenue where we make drm_global_mutex locking (partially) opt-in like
with drm_release_noglobal().

v3:
- Actually push this stuff correctly, don't unlock twice (Chris)
- Fix typo on commit message, plus explain why BKL = drm_global_mutex
  (Sam)

Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200204150146.2006481-5-daniel.vetter@ffwll.ch

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