drm/ast: Try to use MMIO registers when PIO isn't supported

If the PIO resources haven't been assigned, then we have no choice
but try to use the MMIO version. This is the case for example on
POWER8 which doesn't support PIO at all.

Chips rev 0x20 or later have MMIO decoding enabled by default.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>