Emulated coherent graphics memory take 2

The mm patch page walk interface has been reworked to be similar to the reworked page-walk code (mm/pagewalk.c). There have been two other solutions to consider:
1) Using the page-walk code. That is currently not possible since it requires the mmap-sem to be held for the struct vm_area_struct vm_flags and for huge page splitting. The pagewalk code in this patchset can't hold the mmap sems since it will lead to locking inversion.