drm/msm/mdp5: Simplify LM <-> PP mapping

PingPong ID for a Layer Mixer is already contained in
mdp5_hw_mixer.

This avoids the need to retrieve PP ID using macros

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>