drm: fix spelling error in comments

Change the comment typo: "manger" -> "manager".

Signed-off-by: Wang Qing <wangqing@vivo.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1600308275-32094-1-git-send-email-wangqing@vivo.com