[Forward port of a 2.4 patch that got applied last month -- DJ] 
 
AGP/DRM currently assume that GATT entries can be converted
to physical addresses with a simple mask.  Additionally, agpgart
assumes in a couple places that the mask is ~0xfff, i.e., that
all the GART control bits are in the low 12 bits.  Both assumptions
are bogus, so:
  
Make agp_memory.memory[] (exported from agpgart to DRM) contain physical
addresses, not GATT entries.
  
DRM assumes agp_memory contains GATT entries, and it converts them to
physical addresses with "paddr = agp_memory.memory[i] & mask".  460GX
requires both a shift and a mask, so exporting plain physical addresses
and a mask of ~0UL allows agpgart to add 460GX support without requiring
DRM interface changes.