msm: mdss: fix amortized prefill calculations

Fix wrong calculation for the amortized prefill
parameters to use destination coordiates as well
as adjust ts_count denominators for the correct
units, also make sure vsync clock is enabled.
It was causing problem when feature was enabled.

Change-Id: I62a3bd31997be05181de98307089e2a69d98ab7b
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>