IOMX: do not clear buffer if it's allocated by component

The component might depends on their buffers to be initialized
in certain ways to work. Don't clear unless we're allocating it.

bug: 31586647
Change-Id: Ia0a125797e414998ef0cd8ce03672f5b1e0bbf7a
(cherry picked from commit ea76573aa276f51950007217a97903c4fe64f685)
