video: udlfb: Remove redundant gdev variable

gdev is not really needed as the same content can be read
from udev->dev.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Cc: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>