[idd] config: support fractional refresh rates

Store refresh rates in millihertz and preserve three decimal places.

Advertise exact rational rates while accepting legacy integer values.

Accept rates from 23.900 through 1000.000 Hz.

idd/LGCommon/PipeMsg.h

@@ -49,7 +49,7 @@ struct LGPipeMsg
     {
       uint32_t width;
       uint32_t height;
-      uint32_t refresh;
+      uint32_t refreshMilliHz;
     }
     displayMode;
 
@@ -67,4 +67,4 @@ struct LGPipeMsg
     }
     resolutionRejected;
   };
-};
+};