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

Fixes coccicheck warning:

drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c:464:56-57: WARNING comparing pointer to 0
drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c:571:15-16: WARNING comparing pointer to 0
drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c:571:32-33: WARNING comparing pointer to 0
drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c:571:49-50: WARNING comparing pointer to 0
drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c:1968:17-18: WARNING comparing pointer to 0
drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c:1981:17-18: WARNING comparing pointer to 0
drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c:2012:51-52: 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>