drm/ast: Remove reference to struct drm_device.pdev

Using struct drm_device.pdev is deprecated. Upcast with to_pci_dev()
from struct drm_device.dev to get the PCI device structure.

v9:
	* fix remaining pdev references

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Fixes: ba4e0339 ("drm/ast: Fixed CVE for DP501")
Cc: KuoHsiang Chou <kuohsiang_chou@aspeedtech.com>
Cc: kernel test robot <lkp@intel.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Dave Airlie <airlied@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210429105101.25667-2-tzimmermann@suse.de