drm/i915: Reject non-default contexts on non-render again

This reverts the abi-change from

commit 67e3d297
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Fri Dec 6 14:11:01 2013 -0800

    drm/i915: Permit contexts on all rings

We don't actually need this, only the internal changes to allow
contexts on all rings for the purpose of ppgtt switching are required.
And I'm not sure whether this is the right thing to do given some of
the hw features in the pipeline.

Also, new abi needs userspace patches as a proof-of-need, which is
completely lacking here.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>