vp9-rtc: Improve mode check on flat blocks in screen mode

For nonrd-pickmode in screen content mode: modify logic for
inter and intra mode check for spatially flat blocks.
Condition skip of non-zero/zero inter mode based on
zero_temp_sad_source, and force intra/DC check regardless.

Reduces artifacts in scrolling motion.

Change-Id: Iee75cd19d03296afeb649c5bce628806103769ae
1 file changed