drm/ast: Use managed pci functions

Allows us to remove a bit of cleanup code.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "Y.C. Chen" <yc_chen@aspeedtech.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-58-daniel.vetter@ffwll.ch