drm/msm/dpu: fix comparing pointer to 0 in dpu_encoder_phys_cmd.c

Fixes coccicheck warning:

drivers/gpu/drm/msm/disp/dpu1/dpu_encoder_phys_cmd.c:414:52-53: WARNING comparing pointer to 0
drivers/gpu/drm/msm/disp/dpu1/dpu_encoder_phys_cmd.c:443:56-57: WARNING comparing pointer to 0

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>