drm/i915: initialize the context idr unconditionally

It doesn't hurt and it at least prevents us from OOPSing left and
right at quite a few places. This also allows us to simplify the code
a bit by folding the only line of context_open into the callsite.

We obviuosly also need to run the cleanup code unconditionally, too.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>