drm/i915: Remove ctx->last_ring

drm/i915: Remove ctx->last_ring

The original comment that introduced it said:

commit 0009e46cd54324c4af20b0b52b89973b1b914167
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Fri Dec 6 14:11:02 2013 -0800

    drm/i915: Track which ring a context ran on

    Previously we dropped the association of a context to a ring. It is
    however very important to know which ring a context ran on (we could
    have reused the other member, but I was nitpicky).

    This is very important when we switch address spaces, which unlike
    context objects, do change per ring.

    As an example, if we have:

            RCS   BCS
    ctx            A
    ctx      A
    ctx      B
    ctx            B

    Without tracking the last ring B ran on, we wouldn't know to switch the
    address space on BCS in the last row.

But this is not really true, because we are already checking to != from (with
"from" being = ring->last_context) and that should be enough to make sure we
switch to the right address space.

We would have a problem if we switched the context object for every ring (since
then we would fail to do it in some situations) but we only switch it for the
render ring, so we don't care.

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>