video: udlfb: Do not name private data 'dev'

Variable 'dev' is usually used for 'struct device'. Therefore
rename driver private data to dlfb to avoid confusion once
driver will be using dev_*() logging functions.

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