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 <[e-mail]>
Acked-by: Sam Ravnborg <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Acked-by: Thomas Zimmermann <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/gpu/drm-mm.rst

···
 --------------------
 
 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