drm/msm/dsi: Use DIV_ROUND_UP instead of conditional increment on modulo

This exact same math is used to compute bytes_in_slice above in
dsi_update_dsc_timing(), also used to fill slice_chunk_size.

Fixes: b9080324

 ("drm/msm/dsi: add support for dsc data")
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/508935/
Link: https://lore.kernel.org/r/20221026182824.876933-4-marijn.suijten@somainline.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>