drm/msm/dsi: Remove repeated calculation of slice_per_intf

drm/msm/dsi: Remove repeated calculation of slice_per_intf

slice_per_intf is already computed for intf_width, which holds the same
value as hdisplay.

Fixes: 08802f515c3c ("drm/msm/dsi: Add support for DSC configuration")
Reviewed-by: Bjorn Andersson <andersson@kernel.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Vinod Koul <vkoul@kernel.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/508933/
Link: https://lore.kernel.org/r/20221026182824.876933-3-marijn.suijten@somainline.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>