drm/msm/dsi-staging: update frame transfer time calculations

Change updates frame transfer time calculations. Frame threshold
is provided as input to decide on the final transfer time.
Panel dsi clock node followed by mdp transfer time node
will take priority in selecting final transfer time than frame
threshold time.

Change-Id: I40c3abfc635cd9b338b705535612ac32e047ce6e
Signed-off-by: Vara Reddy <varar@codeaurora.org>