drm/i915: Reorganise legacy context switch to cope with late failure

After mi_set_context() succeeds, we need to update the state of the
engine's last_context. This ensures that we hold a pin on the context
whilst the hardware may write to it. However, since we didn't complete
the post-switch setup of the context, we need to force the subsequent
use of the same context to complete the setup (which means updating
should_skip_switch()).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460565315-7748-15-git-send-email-chris@chris-wilson.co.uk