stagefright: fix YV12 chroma stride align for Exynos4

Commit 906eba (exynos4: libstragefright: add support for samsung colorformat/omx/mfc)
was supposed to apply YV12 chroma stride align to 16 for the Planar format but was
mistakenly applied for all the other formats.

Source: http://git.insignal.co.kr/samsung/exynos/android/platform/frameworks/base/tree/media/libstagefright/colorconversion/SoftwareRenderer.cpp?h=exynos-ics&id=ede5597dcf8e9626b6a0ecf1a025c1e4bf57411e#n161

Change-Id: Iae0e785a908be8a452f6c5e947cd0175d777165b