drm/ast: Replace drm_get_pci_device() and drm_put_dev()

Both functions are deprecated. Open-code them them in preparation
of removing struct drm_driver.{load,unload}.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191113155857.9507-2-tzimmermann@suse.de