drm/pcigart: fix the pci gart to use the drm_pci wrapper.

This is the correct fix for the RS690 and hopefully the dma coherent work.

For now we limit everybody to a 32-bit DMA mask but it is possible for
RS690 to use a 40-bit DMA mask for the GART table itself,
and the PCIE cards can use 40-bits for the table entries.

Signed-off-by: Dave Airlie <airlied@redhat.com>