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

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

commit 88733a0b64872357e5ecd82b7488121503cb9cc6 upstream.

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.

Cc: stable@vger.kernel.org
Fixes: d7d0e73f7de33 ("drm/msm/dpu: introduce the dpu_encoder_phys_* for writeback")
Signed-off-by: Nikolay Kuratov <kniv@yandex-team.ru>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/693860/
Link: https://lore.kernel.org/r/20251211093630.171014-1-kniv@yandex-team.ru
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>