Add to_string(DisplayCapability)

For better debugging. This matches other to_string functions, e.g. for
Composition. Use it in the one place I've found a log statement
representing DisplayCapability.

Bug: 193170859
Test: make
Change-Id: Ieb33c5278d2f83f849ca16126230e4dd85c6700e
diff --git a/services/surfaceflinger/DisplayHardware/HWC2.cpp b/services/surfaceflinger/DisplayHardware/HWC2.cpp
index 215d1a3..548d839 100644
--- a/services/surfaceflinger/DisplayHardware/HWC2.cpp
+++ b/services/surfaceflinger/DisplayHardware/HWC2.cpp
@@ -292,9 +292,9 @@
         return mDisplayCapabilities->count(capability) > 0;
     }
 
-    ALOGW("Can't query capability %d."
+    ALOGW("Can't query capability %s."
           " Display Capabilities were not queried from HWC yet",
-          static_cast<int>(capability));
+          to_string(capability).c_str());
 
     return false;
 }