drm/i915: Export intel_context_instance()

We want to pass in a intel_context into intel_context_pin() and that
requires us to first be able to lookup the intel_context!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190426163336.15906-2-chris@chris-wilson.co.uk