drm/i915: Re-arrange execbuf so context is known before engine

Needed for a following patch.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190301140404.26690-23-chris@chris-wilson.co.uk


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>