Document/gpu: Use new vm_fault_t type

We have introduce new return type vm_fault_t for
fault handler. Update the document for the same.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180904041505.GA2712@jordon-HP-15-Notebook-PC