drm/i915: execbuffer pins objects, no need to ensure they're still in the GTT

Before we had the notion of pinning objects, we had a kludge around to make
sure all of the objects were still resident in the GTT before we committed
to executing a batch buffer. We don't need this any longer, and it sticks an
error return in the middle of object domain computations that must be
associated with a subsequent flush/invalidate emmission into the ring.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>