drm/bochs: 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>
Link: http://patchwork.freedesktop.org/patch/msgid/20190723101103.30250-1-hslester96@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>