msm: mdss: use layer mixer number to select pingpong block

In the current implementation, the pingpong block is selected
based on the interface number. This causes issues when a
single DSI command mode panel is connected to Interface #n and
not driven on control path #(n-1). Add change to select pingpong
block based on LM number to handle this.

Change-Id: I195d25bb7dd890c3b731e6ea38b1a41a321b1cf5
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>