drm/msm/sde: add pp1 support for cwb

Present sde driver doesn't support concurrent writeback
for indices less than 2. Add change to support cwb on PP1.

Change-Id: I320ccd35ff3cae577675efb2b68b8a755fdb49d8
Signed-off-by: Shubhashree Dhar <dhar@codeaurora.org>