Documentation: Fix typo error in todo.rst

Change wrong function name drm_modest_lock_all() to drm_modeset_lock_all()

Signed-off-by: Sidong Yang <realwakka@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200528172159.24641-1-realwakka@gmail.com

Documentation/gpu/todo.rst

@@ -305,7 +305,7 @@ acquire context. Replace the boilerplate code surrounding
 drm_modeset_lock_all_ctx() with DRM_MODESET_LOCK_ALL_BEGIN() and
 DRM_MODESET_LOCK_ALL_END() instead.
 
-This should also be done for all places where drm_modest_lock_all() is still
+This should also be done for all places where drm_modeset_lock_all() is still
 used.
 
 As a reference, take a look at the conversions already completed in drm core.