drm/i915: Call context fini at cleanup

If contexts were actually initialized, and we fail somewhere later during
init this would possibly leak memory, and lead to some error messages
about unclean takedown. As the odds of this occurring, and someone
actually caring/noticing are pretty slim, the patch isn't terribly
important.

Found by code inspection while working on something else.

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