drm/fb_helper: Fix a few typos

s/modest/modeset/
s/aftert/after/

Signed-off-by: Lyude <cpaul@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1462375734-8213-3-git-send-email-cpaul@redhat.com