drm/doc: drop struct_mutex references

There's little point in providing partial and ancient information about
the struct_mutex. Some drivers are using it, new ones should not.

As-it this only provides for confusion.
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200515095118.2743122-5-emil.l.velikov@gmail.com

Documentation/gpu/drm-mm.rst

@@ -178,11 +178,8 @@ GEM Objects Lifetime
 --------------------
 
 All GEM objects are reference-counted by the GEM core. References can be
-acquired and release by calling drm_gem_object_get() and drm_gem_object_put()
-respectively. The caller must hold the :c:type:`struct drm_device <drm_device>`
-struct_mutex lock when calling drm_gem_object_get(). As a convenience, GEM
-provides drm_gem_object_put_unlocked() functions that can be called without
-holding the lock.
+acquired and release by calling drm_gem_object_get() and drm_gem_object_put_unlocked()
+respectively.
 
 When the last reference to a GEM object is released the GEM core calls
 the :c:type:`struct drm_driver <drm_driver>` gem_free_object_unlocked