drm/i915: Pass intel_context to intel_context_pin_lock()

Move the intel_context_instance() to the caller so that we can decouple
ourselves from one context instance per engine.

v2: Rename pin_lock() to lock_pinned(), hopefully that is clearer.

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/20190426163336.15906-5-chris@chris-wilson.co.uk