SF: initialize the debug variable for 1035 backdoor

The commit f1833853 unexpectedly removes
initialization of the variable mDebugDisplayModeSetByBackdoor, which is
needed to ensure 1035 backdoor transaction always be effective without
depending on previous call result.

Bug: 192129895
Test: service call SurfaceFlinger 1035 i32 <mode> i64 <display-id>

Signed-off-by: Shiyong Li <shiyongli@google.com>
Change-Id: I7a027b9db51389f48cf0745eb0adde8b71312e1b