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

drivers/gpu/drm/i915/intel_display.c

@@ -481,6 +481,7 @@ skl_wa_827(struct drm_i915_private *dev_priv, int pipe, bool enable)
 {
 	if (enable)
 		I915_WRITE(CLKGATE_DIS_PSL(pipe),
+			   I915_READ(CLKGATE_DIS_PSL(pipe)) |
 			   DUPS1_GATING_DIS | DUPS2_GATING_DIS);
 	else
 		I915_WRITE(CLKGATE_DIS_PSL(pipe),