drm/ttm: TTM fault handler helpers

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.

Cc: "Christian König" <christian.koenig@amd.com>

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: "Christian König" <christian.koenig@amd.com> #v1