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/HWC2.h b/services/surfaceflinger/DisplayHardware/HWC2.h
index 532685c..900b3d7 100644
--- a/services/surfaceflinger/DisplayHardware/HWC2.h
+++ b/services/surfaceflinger/DisplayHardware/HWC2.h
@@ -218,10 +218,8 @@
             std::unordered_map<Layer*, Composition>* outTypes);
     [[clang::warn_unused_result]] Error getColorModes(
             std::vector<android::ui::ColorMode>* outModes) const;
-    // outSupportedPerFrameMetadata is an opaque bitmask to the callers
-    // but contains HdrMetadata::Type::*.
-    [[clang::warn_unused_result]] Error getSupportedPerFrameMetadata(
-            int32_t* outSupportedPerFrameMetadata) const;
+    // Returns a bitmask which contains HdrMetadata::Type::*.
+    [[clang::warn_unused_result]] int32_t getSupportedPerFrameMetadata() const;
     [[clang::warn_unused_result]] Error getRenderIntents(
             android::ui::ColorMode colorMode,
             std::vector<android::ui::RenderIntent>* outRenderIntents) const;