surfaceflinger: do not propogate getPerFrameMetadataKeys errors

We don't want to burden the callers to handle the errors.  This also
silences

  E HWComposer: getSupportedPerFrameMetadata: getSupportedPerFrameMetadata failed for display 0: Unsupported (8)

on unsupported devices.

Test: no error logged
Change-Id: I05cb555a5405e330d22008f32a16e4c4b816d721
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.cpp b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
index 8b7bca7..bf8905a 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.cpp
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
@@ -745,12 +745,7 @@
 
 int32_t HWComposer::getSupportedPerFrameMetadata(int32_t displayId) const {
     RETURN_IF_INVALID_DISPLAY(displayId, 0);
-
-    int32_t supportedMetadata;
-    auto error = mDisplayData[displayId].hwcDisplay->getSupportedPerFrameMetadata(
-            &supportedMetadata);
-    RETURN_IF_HWC_ERROR(error, displayId, 0);
-    return supportedMetadata;
+    return mDisplayData[displayId].hwcDisplay->getSupportedPerFrameMetadata();
 }
 
 std::vector<ui::RenderIntent> HWComposer::getRenderIntents(int32_t displayId,