msm: mdss: Choose the correct PPB for right only update in non-DSC mode

When performing right-only update on a dual LM single display mode with
3D Mux enabled, the right mixer's ping pong buffer should be used.

Change-Id: Ib530d7503ca46a01198cc88de5312635bd84a6fb
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>