drm/msm/dsi: add a comment to explain pkt_per_line encoding

Make it clear why the pkt_per_line value is being "divided by 2".

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Jun Nie <jun.nie@linaro.org>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8550-QRD
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8650-QRD
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8650-HDK
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/596236/
Link: https://lore.kernel.org/r/20240530-msm-drm-dsc-dsi-video-upstream-4-v6-6-2ab1d334c657@linaro.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>