With the vmwgfx dirty tracking, the default TTM fault handler is not
completely sufficient (vmwgfx need to modify the vma->vm_flags member,
and also needs to restrict the number of prefaults).
We also want to replicate the new ttm_bo_vm_reserve() functionality
So start turning the TTM vm code into helpers: ttm_bo_vm_fault_reserved()
and ttm_bo_vm_reserve(), and provide a default TTM fault handler for other
drivers to use.