drm/i915: Make context pinning part of intel_context_ops

Push the intel_context pin callback down from intel_engine_cs onto the
context itself by virtue of having a central caller for
intel_context_pin() being able to lookup the intel_context itself.

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/20190308132522.21573-5-chris@chris-wilson.co.uk