video: fbdev: Use dev_get_drvdata()

Using dev_get_drvdata directly.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Wan ZongShun <mcuos.com@gmail.com>
Cc: Kukjin Kim <kgene@kernel.org>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Michal Januszewski <spock@gentoo.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>