drm/i915: Allow the user to pass a context to any ring

With full-ppgtt, we want the user to have full control over their memory
layout, with a separate instance per context. Forcing them to use a
shared memory layout for !RCS not only duplicates the amount of work we
have to do, but also defeats the memory segregation on offer.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20160822080350.4964-4-chris@chris-wilson.co.uk


Reviewed-by: John Harrison <john.c.harrison@intel.com>
Reviewed-by: Thomas Daniel <thomas.daniel@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>