sdm: Clear cached panel brightness settings

Clear cached panel brightness settings when SetPanelBrightness()
is successfully called to fix the issue that the brithtness of
power on mode is overwritten by the cached brightness.

Change-Id: I93efb4e1087892f0c63eaa83fb008bc5241c1295
diff --git a/sdm/libs/core/display_builtin.cpp b/sdm/libs/core/display_builtin.cpp
index b4628aa..75a5046 100644
--- a/sdm/libs/core/display_builtin.cpp
+++ b/sdm/libs/core/display_builtin.cpp
@@ -565,6 +565,7 @@
   DisplayError err = hw_intf_->SetPanelBrightness(level);
   if (err == kErrorNone) {
     level_remainder_ = level_remainder;
+    pending_brightness_ = false;
     DLOGI_IF(kTagDisplay, "Setting brightness to level %d (%f percent)", level,
              brightness * 100);
   } else if (err == kErrorDeferred) {