drm/msm/dsi-staging: Update pclk calculation stored in mode

pclk is udpated in mode which is used by usermode. Update
the calculation as per new recommendation.

Change-Id: Iee074cfdbc9a61f3587c5d36edcad0c54920464b
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>