i915: Fix up ring initialization to cover G45 oddities

G45 appears quite sensitive to ring initialization register writes,
sometimes leaving the HEAD register with the START register contents. Check
to make sure HEAD is reset correctly when START is written, and fix it up,
screaming loudly.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>