drm/msm/dsi: add a helper method to compute the dsi byte clk

drm/msm/dsi: add a helper method to compute the dsi byte clk

Re-arrange the dsi_calc_pclk method to two helpers, one to
compute the DSI byte clk and the other to compute the pclk.

This makes the separation of the two clean and also allows
clients to compute and use the dsi byte clk separately.

changes in v2:
	- move the assignments to definition lines

Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/518005/
Link: https://lore.kernel.org/r/20230112001600.12791-1-quic_abhinavk@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>