drm/i915/icl: Fix power well 2 wrt. DC-off toggling order

To enable DC5/6 power well 2 has to be disabled as for previous
platforms, so fix things up.

Bspec: 4234
Fixes: 67ca07e7 ("drm/i915/icl: Add power well support")
Cc: Animesh Manna <animesh.manna@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
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: https://patchwork.freedesktop.org/patch/msgid/20181102182200.17219-1-imre.deak@intel.com