hwc: Set HPD once at boot up

- HPD has to be set only once during boot up.
- This also helps in use case where HPD is turned off
  (on purpose)through by adb shell for power testing
  and on suspend/resume, it gets re-enabled.

Change-Id: I7bf6fdd08e719432029c5d6e5756685d36cdca43
diff --git a/libhwcomposer/hwc.cpp b/libhwcomposer/hwc.cpp
index c96eb1e..22b576e 100644
--- a/libhwcomposer/hwc.cpp
+++ b/libhwcomposer/hwc.cpp
@@ -475,10 +475,11 @@
                 return -errno;
             }
 
-            if(mode == HWC_POWER_MODE_NORMAL) {
+            if(mode == HWC_POWER_MODE_NORMAL && !ctx->mHPDEnabled) {
                 // Enable HPD here, as during bootup POWER_MODE_NORMAL is set
                 // when SF is completely initialized
                 ctx->mHDMIDisplay->setHPD(1);
+                ctx->mHPDEnabled = true;
             }
 
             ctx->dpyAttr[dpy].isActive = not(mode == HWC_POWER_MODE_OFF);