drm/i915: Setup the RCS ring prior to execution

We need to set the various ring registers prior to restarting the
engine, or else we may restart it after reset/resume in an ill-defined
state.

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190418132720.3716-2-chris@chris-wilson.co.uk