drm/atomic: fix doc to use new name for commit types

Use "non-blocking" and "blocking" instead of old names.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170427143506.32650-1-gustavo@padovan.org