drm/atomic-helper: Fix typo

Fix "esay-to-use" to "easy-to-use" typo.

Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171012140616.9002-1-thierry.reding@gmail.com