hwc: Enable downscale feature through system property

Add support to turn the MDP downscale feature ON through the
use of a system property. 'sys.hwc.mdp_downscale_enabled' shall be
set to 'true' in build.prop file to enable this functionality.

This is required for OEMs that might have privacy concerns over panel
size information being exposed.

Change-Id: I354b25b1fdabe5786aed7552f2afccda35412467
diff --git a/libhwcomposer/hwc_utils.h b/libhwcomposer/hwc_utils.h
index 8293505..660916d 100644
--- a/libhwcomposer/hwc_utils.h
+++ b/libhwcomposer/hwc_utils.h
@@ -516,6 +516,9 @@
     bool mPanelResetStatus;
     // number of active Displays
     int numActiveDisplays;
+    // Downscale feature switch, set via system the property
+    // sys.hwc.mdp_downscale_enabled
+    bool mMDPDownscaleEnabled;
 };
 
 namespace qhwc {