msm: mdss: half the y offset value for s/w deinterlace

S/W deinterlaces interleaved videos by changing the original height to
the half. Y offset needs to be half as well and this half y should be
checked again to make sure it is not an odd value, which is required
by mdp h/w.

CRs-fixed: 524678
Change-Id: I429dd6e66a8c034815ab3751c31e7e60c25a8051
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>