drm/msm: fix returnvar.cocci warning

Fix the following coccicheck warning:
drivers/gpu/drm/msm/disp/mdp4/mdp4_kms.c:24:5-8: Unneeded variable:
"ret". Return "0" on line 75.

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/483328/
Link: https://lore.kernel.org/r/20220425122223.7415-1-guozhengkui@vivo.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>