drm/i915: Change i915_gem_fault() to return vm_fault_t

In preparation for vm_fault_t becoming a distinct type, convert the
fault handler (i915_gem_fault()) over to the new interface.

Based on a patch by Souptick Joarder

References: 1c8f4220 ("mm: change return type to vm_fault_t")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Souptick Joarder <jrdr.linux@gmail.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180606214520.20220-1-chris@chris-wilson.co.uk