drm/radeon/nouveau: fix build regression on alpha due to Xen changes.

The Xen changes were using DMA_ERROR_CODE which isn't defined on a few
platforms, however we reverted the Xen patch that caused use to try and
use this code path earlier in 2.6.39 cycle, so for now lets just force
the code to never take this path and allow it to build again on alpha.

The proper long term answer is probably to store if the dma_addr has
been assigned to alongside the dma_addr in the higher level code,
though I think Thomas wanted to rewrite most of this anyways properly.

Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>