drm/i915: Only flush fbc on sw when fbc is enabled.

Avoid touching fbc register when fbc is disabled.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NRodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -352,6 +352,9 @@ void gen8_fbc_sw_flush(struct drm_device *dev, u32 value)
 	if (!IS_GEN8(dev))
 		return;
 
+	if (!intel_fbc_enabled(dev))
+		return;
+
 	I915_WRITE(MSG_FBC_REND_STATE, value);
 }