drm/ast: Fix AST2400 POST failure without BMC FW or VBIOS

The current POST code for the AST2300/2400 family doesn't work properly
if the chip hasn't been initialized previously by either the BMC own FW
or the VBIOS. This fixes it.

Signed-off-by: Y.C. Chen <yc_chen@aspeedtech.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Tested-by: Y.C. Chen <yc_chen@aspeedtech.com>
Acked-by: Joel Stanley <joel@jms.id.au>
Cc: <stable@vger.kernel.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>