drm/i915/ctx: Remove bad invariant

It's not that the assertion is incorrect, but rather that we can call
do_destroy early in loading, and we will falsely BUG().

Since contexts have been in for a while now, and in the internal APIs
are pretty stable, it should be fairly safe to remove this.

v2: Remove unused dev_priv, and dev

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