drm/tegra: Adding new typedef vm_fault_t

Use new return type vm_fault_t for fault handler. For now, this is just
documenting that the function returns a VM_FAULT value rather than an
errno. Once all instances are converted, vm_fault_t will become a
distinct type.

Reference id -> 1c8f4220 ("mm: change return type to vm_fault_t")

Previously vm_insert_page() returns err which driver mapped into
VM_FAULT_* type. The new function vmf_insert_page() will replace this
inefficiency by returning VM_FAULT_* type.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>