msm: vidc: Modify the MPEG-2 aspect ratio calculation

MPEG-2 display aspect ratio calculation was done similar
to MPEG-4 standard. Correct it as per the MPEG-2 standard
and video core interface specification.

Change-Id: I1aa98ca238c7b38e35c78905b29457cfa59edd45
Signed-off-by: Shobhit Pandey <cshopan@codeaurora.org>