drm/i915: Reject UNSYNCHRONIZED for userptr, v2.

We should not allow this any more, as it will break with the new userptr
implementation, it could still be made to work, but there's no point in
doing so.

Inspection of the beignet opencl driver shows that it's only used
when normal userptr is not available, which means for new kernels
you will need CONFIG_I915_USERPTR.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210323155059.628690-15-maarten.lankhorst@linux.intel.com