drm/doc: Fix indenting in drm_modeset_lock.c comment

This isn't part of the code snippet anymore ...
Acked-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161129092440.6940-1-daniel.vetter@ffwll.ch