msm: mdss: Fix writeback MISR for apq8084

Layer Mixer MISR register configuration has moved to upperpipe
MISR Block in apq8084. Address the same by properly taking
care of Register offsets.

Change-Id: I1c6612e833c17f0fa72f20b0bc7aa0c5a7e67228
Signed-off-by: Vishnuvardhan Prodduturi <vproddut@codeaurora.org>