drm/msm/sde: programmable pre-fetch support for video encoders

Add support in encoder for programming early fetch in the
vertical front porch.

Change-Id: I60fcf4a4e6aea80292b590ee14506579123f372d
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>
Signed-off-by: Krishna Srinivas Kundurthi <kskund@codeaurora.org>