drm/i915: Fix the inconsistent RMW in WA 827

RMW is used only in the disable path. Using it in enable path
for consistency.

Suggested-by: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190330011921.10397-2-radhakrishna.sripada@intel.com