SurfaceTexture: Fix to return the oldest of free buffers to Client on Dequeue call

Surface Texture dequeue logic is modified to return the oldest of the
free buffers to Client on dequeue call.

Currently dequeue method is returning the first buffer index which is free.
The parsing is done in ascending order of the buffer slot indices.
This leads to returning the buffer which has been just queued to composer,
and hence display, and this defeats the purpose of having minimum dequeue count
as 2 in asynchrnouse mode.

This is fixed by checking all the free slots and returning the oldest buffer.

Change-Id: Ibbac10593c3994c278c601af0480b171635ecdd4
Signed-off-by: Sunita Nadampalli <sunitan@ti.com>