disp: msm: dsi: cphy constant fps porch calculation

This change adds porch calculation support to maintain
constant fps during clock switch for CPhy.

Change-Id: I74b2f0e064441fba7f452b06438ece7fe3b373eb
Signed-off-by: Ritesh Kumar <riteshk@codeaurora.org>
Signed-off-by: Harigovindan P <harigovi@codeaurora.org>