drm/msm/dpu: Add missing NULL pointer check for pingpong interface

In the Linux kernel, the following vulnerability has been resolved:

drm/msm/dpu: Add missing NULL pointer check for pingpong interface

It is checked almost always in dpu_encoder_phys_wb_setup_ctl(), but in a

single place the check is missing.

Also use convenient locals instead of phys_enc->* where available.

Patchwork: https://patchwork.freedesktop.org/patch/693860/