pyxis: Add panel property

Fingerprint HAL looks for sys.panel.touch_vendor:
E [GF_HAL][ExtModuleCreator]: Failed to getprop sys.panel.touch_vendor

On stock ROM this property is set by init.panel_info.sh which is not
shipped. But on Pyxis the value is always '3' so we can add it
statically into system.prop.

Change-Id: I9a70656f2b410d622976abb72e324aa3333aa390

system.prop

@@ -6,3 +6,6 @@ persist.vendor.camera.multicam=TRUE
 persist.vendor.camera.multicam.fpsmatch=TRUE
 persist.vendor.camera.multicam.framesync=1
 persist.vendor.camera.multicam.hwsync=TRUE
+
+# Panel
+sys.panel.touch_vendor=3