msm: mdss: map writeback buffer to correct context bank

Writeback uses rotation smmu on thulium target for buffer
mapping. Selecting wrong context bank is leading to
writeback failure on this target. This change fixes the
context bank selection for writeback use cases.

Change-Id: I23369bb43cc3fd05457fb003ee58bba21eeaa192
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>