drm/i915: print warning if vmi915_gem_fault error is not handled

Falling into default case in vmi915_gem_fault is a bug. Be more
verbose about it.

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>