drm/cirrus: Remove references to struct drm_device.pdev

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

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201201103542.2182-6-tzimmermann@suse.de