drm/msm/dsi: improve/fix dsc pclk calculation

drm_mode_vrefresh() can introduce a large rounding error, avoid it.

Fixes: 7c9e4a55 ("drm/msm/dsi: Reduce pclk rate for compression")
Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/618432/
Link: https://lore.kernel.org/r/20241007050157.26855-1-jonathan@marek.ca


Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>