disp: msm: sde: remove idle time from Qsync threshold calculation

Remove idle time from the Qsync calculation, round down to
the nearest multiple of 4 and remove a couple of extra lines
to compensate for possible latencies. This helps to reduce
the possibility of causing tearing due to the driver calculation
mismatching with the DDIC's expected timeout value.

Change-Id: I8a53a989e26cbd7f0e2b94caa8df8f5bee3ad26c
Signed-off-by: Steve Cohen <cohens@codeaurora.org>