drm/radeon/alpha: Add Alpha support to Radeon DRM code

Alpha needs to have available the system bus address for the Radeon's
local memory, so that it can be used in ttm_bo_vm_fault(), when building
the PTEs for accessing that VRAM.  So, we make bus.addr hold the ioremap()
return, and then we can modify bus.base appropriately for use during page
fault processing.

Signed-off-by: Jay Estabrook <jay.estabrook@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>