OMXNodeInstance: Don't perform buffer alignment on mediatek hardware

The node instance was aligning buffers for all node types in mediatek
hardware. However, when there is a fallback to a software codec
(such as the case of ffmpeg), the client does not allocate an aligned
buffer and by aligning just the size here, we are potentially corrupting
an area of memory that is owned by some other component.

Moreso, this alignment is not needed, since the mtk buffer allocator
already aligns the size of buffers when they are requested.

Change-Id: I621085edb0766fb2e8da1c008a56eef1c991fdd4