drm/i915: More use of the cached LRC state

Since:

commit 82352e90
Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Date:   Fri Jan 15 17:12:45 2016 +0000

    drm/i915: Cache LRC state page in the context

and:

commit 0eb973d3
Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Date:   Fri Jan 15 15:10:28 2016 +0000

    drm/i915: Cache ringbuffer GTT VMA

We can also remove the ring buffer start updates on every
context update since the address will not change for the
duration of the LRC pin.

For GuC we can remove the update altogether because it
only cares about the ring buffer start.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Alex Dai <yu.dai@intel.com>
Cc: Dave Gordon <david.s.gordon@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1453466567-33369-1-git-send-email-tvrtko.ursulin@linux.intel.com