drm/i915: Set up mocs tables before restarting the engines

After a reset, we may immediately begin executing requests on restarting
the engines. Ergo this has to be last step with all re-initialisation
completed beforehand. The mocs setup was after we started executing the
requests; do it earlier!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171102131430.22328-1-chris@chris-wilson.co.uk