drm/savage: Remove references to struct drm_device.pdev

Replace all references to struct drm_device's pdev field with
an upcast from dev.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210502104953.21768-5-tzimmermann@suse.de