staging: drm/imx: drop "select OF_VIDEOMODE"

Commit ac4c1a9b ("staging: drm/imx: Add LDB support") added the
DRM_IMX_LDB Kconfig entry. That entry selects OF_VIDEOMODE. But there is
no Kconfig symbol named OF_VIDEOMODE. The select statement for that
symbol is a nop. Drop it.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>