drm/radeon: Fix pci_map_page() error checking

0 is a valid DMA address from pci_map_page(), use pci_dma_mapping_error()
instead to check for errors

[airlied: fix warning + two other places with errors.]

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