drm/ast: Fix missing conversions to managed API

The commit 7cbb93d8 ("drm/ast: Use managed pci functions")
converted a few PCI accessors to the managed API and dropped the
manual pci_iounmap() calls, but it seems to have forgotten converting
pci_iomap() to the managed one.  It resulted in the leftover resources
after the driver unbind.  Let's fix them.

Fixes: 7cbb93d8 ("drm/ast: Use managed pci functions")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421170458.21178-1-tiwai@suse.de