disp: msm: sde: only flush SSPPs when enabling sys-cache read

The worker function for enabling sys-cache re-uses the flushes
from the previous commit. This ends up flushing several
blocks which have no programming update. Instead, only flush
planes attached to the CRTC which is entering sys-cache state.

Change-Id: Ieed68a713e9e56a5718518cf1a6b632f37864fb9
Signed-off-by: Steve Cohen <cohens@codeaurora.org>