drm/msm: fix warning "using plain integer as NULL pointer"

Fix sparse warning:
drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c:1195:41: warning: Using plain integer as NULL pointer
drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c:1348:32: warning: Using plain integer as NULL pointer

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Link: https://lore.kernel.org/r/20210510031250.5303-1-bernard@vivo.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>