drm/gem: support BO freeing without dev->struct_mutex

Finally all the core gem and a lot of drivers are entirely free of
dev->struct_mutex depencies, and we can start to have an entirely
lockless unref path.

To make sure that no one who touches the core code accidentally breaks
existing drivers which still require dev->struct_mutex I've made the
might_lock check unconditional.

While at it de-inline the ref/unref functions, they've become a bit
too big.

v2: Make it not leak like a sieve.

v3: Review from Lucas:
- drop != NULL in pointer checks.
- fixup copypasted kerneldoc to actually match the functions.

v4:
Add __drm_gem_object_unreference as a fastpath helper for drivers who
abolished dev->struct_mutex, requested by Chris.

v5: Fix silly mistake in drm_gem_object_unreference_unlocked caught by
intel-gfx CI - I checked for gem_free_object instead of
gem_free_object_unlocked ...

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de> (v3)
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v4)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1462178451-1765-1-git-send-email-daniel.vetter@ffwll.ch