drm/msm/sde: remember topology to disable ppsplit correctly

Add memory of the topology in the previous atomic state to
properly handle disabling of ping pong split topology which
doesn't register the slave encoder for IRQs.

Change-Id: I3ff1bc470713721149d90d462bfb97d8beb55f95
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>