msm: mdss: enable overfetch in dual pipe scenarios

Overfetch is disabled on the borders of videos to avoid picking up
padding which may result in greenish border on right and bottom. However
this has side effect for dual pipe cases where overfetch is required for
smooth scaling effect in the middle of image. Add a flag to catch these
cases and enable overfetching in the middle.

Change-Id: I2545dbdab2c0643986fefa85990e9e820e4c3611
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>