drm/omap: fix typo

Fix spelling mistake: "lenght" -> "length"

Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190107130934.9997-1-mcroce@redhat.com