drm/msm: De-open-code some CP_EVENT_WRITE

Replace some open coding to improve readability.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/499272/
Link: https://lore.kernel.org/r/20220821155441.1092134-1-robdclark@gmail.com