drm/i915: add Ivybridge clock gating init function

Some of the bits have changed, including one we were setting that enables
a VGA test mode, preventing pipe B from working at all.  So add a new
IVB specific function with the right bits.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>