nouveau/ttm/PCIe: Use dma_addr if TTM has set it.

If the TTM layer has used the DMA API to setup pages that are
TTM_PAGE_FLAG_DMA32 (look at patch titled: "ttm: Utilize the
DMA API for pages that have TTM_PAGE_FLAG_DMA32 set"), lets
use it when programming the GART in the PCIe type cards.

This patch skips doing the pci_map_page (and pci_unmap_page) if
there is a DMA addresses passed in for that page. If the dma_address
is zero (or DMA_ERROR_CODE), then we continue on with our old
behaviour.

[v2: Added a review-by tag]

Reviewed-by: Thomas Hellstrom <thomas@shipmail.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Tested-by: Ian Campbell <ian.campbell@citrix.com>