drm_hwcomposer: Clear staged_mode_config_id_ after SetConfig

It's possible that a config change could be queued to happen at the next
composition, but that composition doesn't happen. In such a case,
staged_mode_config_id_ remains set.

If a blocking SetConfig call happens, clear staged_mode_config_id_ so
that ensuing calls to getActiveConfig will not erroneously return the
stale staged_mode_config_id_.

Change-Id: I087eea140f3c02dd0c775ab742ad2d511a0e66cc
Signed-off-by: Drew Davenport <ddavenport@google.com>
diff --git a/hwc2_device/HwcDisplay.cpp b/hwc2_device/HwcDisplay.cpp
index 43564d4..716c33a 100644
--- a/hwc2_device/HwcDisplay.cpp
+++ b/hwc2_device/HwcDisplay.cpp
@@ -231,6 +231,7 @@
 
   ALOGV("Blocking config succeeded.");
   configs_.active_config_id = config;
+  staged_mode_config_id_.reset();
   return ConfigError::kNone;
 }