drm/i915: Pin relocations for the duration of constructing the execbuffer

As the execbuffer dispatch grows ever more complex and involves multiple
stages of moving objects into the aperture, we need to take greater care
that we do not evict our execbuffer objects prior to dispatch. This is
relatively simple as we can just keep the objects pinned for not just
the relocation but until we are finished.

One such example is the possibility of the context switch causing an
eviction or hitting the shrinker in order to fit its object into the
aperture.

Link: http://lists.freedesktop.org/archives/intel-gfx/2013-November/036166.html


Reported-by: "Siluvery, Arun" <arun.siluvery@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: stable@vger.kernel.org
Acked-by: Ben Widawsky <ben@bwidawsk.net>
Tested-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Add the additional explanations from Chris to the commit
message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>