drm/drv: Make use of local variable driver in drm_dev_register()

There is a local variable that contains dev->driver. Make use of it
instead of "open coding" it.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221219183147.1639399-1-u.kleine-koenig@pengutronix.de