drm/i915: Treat using a purged buffer as a source of EFAULT

Since a purged buffer is one without any associated pages, attempting to
use it should generate EFAULT rather than EINVAL, as it is not strictly
an invalid parameter.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>