drm/i915: Clear self-refresh watermarks when disabled

If we elect to disable self-refresh as they require too many FIFO
entries, clear the values prior to writing them into the registers. If
they are too large they may occupy more bits than available and so
corrupt neighbouring WM values.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>