Make life easier in upcoming patches by moving the context_pin and
context_unpin vfuncs into inline helpers.

v2: Fixup mock_engine to mark the context as pinned on use.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180430131503.5375-2-chris@chris-wilson.co.uk