msm: mdss: add fudge factor to ib for single interface displays

For single interface video mode displays, the utilization
of the ddr channels is not as efficient as the dual interface
displays, and in some scenarios this efficiency gets reduced
by half. This is valid when display driver is doing an honest
voting and causes underruns in multiple scenarios.
Therefore this change adds a factor that account for those
inefficiencies, specific to single interface controllers.

Change-Id: I925db9defa16346c4400732310dbb679458dd0dd
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>
Signed-off-by: Anusha Koduru <kanusha@codeaurora.org>