drm/nouveau: Remove references to struct drm_device.pdev

Using struct drm_device.pdev is deprecated. Convert nouveau to struct
drm_device.dev. No functional changes.

v3:
	* fix nv04_dfp_update_backlight() as well (Jeremy)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jeremy Cline <jcline@redhat.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210107080748.4768-8-tzimmermann@suse.de