drm/i915: Silence the WARN if the user tries to GTT mmap an incoherent object

If the user tries to mmap through the GTT an object that is marked as
snooped, we report an error rather than allow the GPU to hang the
machine. The choice of EINVAL, however, was unfortunate as we turn that
into a WARN rather than a quiet SIGBUS.

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