drm: Remove unneeded dma sync in ATI pcigart alloc

Now that the ATI pcigart code uses dma_alloc_coherent, we don't need
the dma_sync_single_for_device() that we used to have here.

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