drm/omap: drop incorrect comment

The wrappers have been removed in commit 5a35876e
(drm: omapdrm: Remove manual update display support)
and will not be reintroduced, since the normal sys
functions properly call the dirty callback.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>