drm/msm/dsi: Document DSC related pclk_rate and hdisplay calculations

Provide actual documentation for the pclk and hdisplay calculations in
the case of DSC compression being used.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/577534/
Link: https://lore.kernel.org/r/20240208-fd_document_dsc_pclk_rate-v4-1-56fe59d0a2e0@linaro.org