HWC2: Check SidebandStream capability on load

Checks whether the HWC2 device has the SidebandStream capability
before attempting to load the corresponding function pointer

Bug: 28161394
Change-Id: I6407d61a1b23138781e57213bcb868be46609018
diff --git a/services/surfaceflinger/DisplayHardware/HWC2.h b/services/surfaceflinger/DisplayHardware/HWC2.h
index 7d33a0a..967add0 100644
--- a/services/surfaceflinger/DisplayHardware/HWC2.h
+++ b/services/surfaceflinger/DisplayHardware/HWC2.h
@@ -89,6 +89,8 @@
     // as connected
     std::shared_ptr<Display> getDisplayById(hwc2_display_t id);
 
+    bool hasCapability(HWC2::Capability capability) const;
+
 private:
     // Initialization methods