video: fbdev: radeonfb: Use dev_get_drvdata

Instead of using to_pci_dev + pci_get_drvdata,
use dev_get_drvdata to make code simpler.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[b.zolnierkie: fix patch summary and intendation]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190724131900.2039-1-hslester96@gmail.com