drm/nouveau/fb/nv50: defer DMA mapping of scratch page to oneinit() hook

The 100c08 scratch page is mapped using dma_map_page() before the TTM
layer has had a chance to set the DMA mask. This means we are still
running with the default of 32 when this code executes, and this causes
problems for platforms with no memory below 4 GB (such as AMD Seattle)

So move the dma_map_page() to the .oneinit hook, which executes after the
DMA mask has been set.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>