drm/ast: Remove references to struct drm_device.pdev

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

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201201103542.2182-4-tzimmermann@suse.de