drm/msm: remove unneeded variable

Fix the following coccicheck REVIEW:
./drivers/gpu/drm/msm/edp/edp_ctrl.c:1245:5-8 Unneeded variable

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Chi Minghao <chi.minghao@zte.com.cn>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20210831115127.18236-1-chi.minghao@zte.com.cn


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