drm/i915/vlv: Fix port B PLL opamp initialization

The current code looks like a typo, the specification calls for setting
bits 31:24 to 0x8C, while preserving bits 23:0. Fix things accordingly.

I'm not aware of the typo causing a real problem, so the fix is only for
consistency.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1494408113-379-1-git-send-email-imre.deak@intel.com