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: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
index 675e8a2..6f3b94b 100644
--- a/drivers/gpu/drm/i915/intel_pm.c
+++ b/drivers/gpu/drm/i915/intel_pm.c
@@ -352,6 +352,9 @@
 	if (!IS_GEN8(dev))
 		return;
 
+	if (!intel_fbc_enabled(dev))
+		return;
+
 	I915_WRITE(MSG_FBC_REND_STATE, value);
 }