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 ea76573a)
(cherry picked from commit 640b04121d7cd2cac90e2f7c82b97fce05f074a5)