gpu: drm: vgem: Change return type to vm_fault_t

Use new return type vm_fault_t for fault handler.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180416150232.GA26745@jordon-HP-15-Notebook-PC