drm/i915/ringbuffer: 2-step restart

We may be simply restarting too fast for the culmudgeonly gen3/gen4 as
we still see missing interrupts following a reset. So let's try
restarting a little slower, first wake up the ring empty and then tell
it about the work it has to perform.

References: https://bugs.freedesktop.org/show_bug.cgi?id=108735


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181126122821.4537-1-chris@chris-wilson.co.uk