ati_radeon: Support PCI virtual not eq bus mapping.

Use pci_bus_to_virt() to convert the bus address from the BARs to
virtual address' to eliminate the direct mapping requirement.

Rename variables to better match usage (_phys -> _bus or no-suffix)

This fixes the mpc8572ds CONFIG_PHYS_64BIT mode failure:
"videoboot: Video ROM failed to map!"

Tested on mpc8572ds with and without CONFIG_PHYS_64BIT.

Signed-off-by: Ed Swarthout <Ed.Swarthout@freescale.com>