drm/msm/dpu: Fix slice_last_group_size calculation

drm/msm/dpu: Fix slice_last_group_size calculation

Correct the math for slice_last_group_size so that it matches the
calculations downstream.

Fixes: c110cfd1753e ("drm/msm/disp/dpu1: Add support for DSC")
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/539269/
Link: https://lore.kernel.org/r/20230329-rfc-msm-dsc-helper-v14-7-bafc7be95691@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>