drm/i915: No need to power up PG2 for GMBUS on BXT

GMBUS lives in PG1, so no need to power up PG2. We do want to prevent
the DMC from making a mess of things though, so add GMBUS to the DC off
power well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171208213739.16388-2-ville.syrjala@linux.intel.com
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>