msm: mdss: fix sw calculation of interlace x offset

Interlacing is handled in software by changing the original height to
half which would become the width after rotation. In cases of 90 degree
rotation the height becomes the displayed width so in that case the
source x offset needs to be updated to half of intended similar to
source width.

CRs-Fixed: 545926
Change-Id: I0de9adb243d6162f5387b1ebc2821141eea4b111
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>