vp9-screen: Fix to screen wth layered encoding

vp9-screen: Fix to screen wth layered encoding

zero_temp_sad_source is only computed when
compute_source_sad_onepass and sf->use_source_sad are
on, which currently is only for the top layer of the
layered encoding. So qualify the usage of
zero_temp_sad_source on those flags.

This affects the quality/speed of the lower layers of
screen content mode when SVC (quality layers) are used.

Change-Id: I54167265a05a4b918ce015931375aa42d3e75cf5