auxdisplay: simplify getting .drvdata

We should get drvdata from struct device directly. Going via
platform_device is an unneeded step back and forth.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>