msm: mdss: fix underruns during hflip and downscaling scenarios

Within the mdp clock calculations, we need to account for
some HW inefficiencies when a high downscaling and hflip
are enabled. This change adds a fudge factor to account
for these inefficiencies, fixing some underruns observed
in these scenarios.

Change-Id: I43d3ecfa5b05d0af30c98505da1a2736ff6f50aa
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>