drm/msm/sde: consider transfer time factor in pclk calculation

In certain panels with low porch values, the current calculated
pclk is less MDP clock value when transfer time is
considered in command mode. This was leading to higher ping pong
transfer time. New changes made takes this factor into
consideration in pclk calculation in command mode.

Change-Id: I1a8dcac187f26e5b547754199c1c5a98346d9656
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>