msm: mdss: Initialize right mixer ROI for split control

This change initializes ROI of the right mixer on dual DSI panels
to its complete resolution. This will ensure that the right mixer
composes to its full resolution when partial update feature is not
enabled / applicable.

CRs-Fixed: 556185
Change-Id: I3c2f2b9a220dc223ef66475d6f6cbfc18c4a67db
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>