mm-video-v4l2: venc: Correct buffer index selection

meta_mode_enable reflects whether the component is using meta
buffers or the input mem pointer. Use this flag (instead of
mUseProxyColorFormat) when computing the buffer index to ensure
that a valid index is selected.

For non-color-conversion input buffers, the component should return
IL client buffers instead of meta buffers. For opaque input buffers
(which also do not require color-conversion), return meta buffers to
the base layer instead of client buffers.

b/17664960

Author: <lenhardw@codeaurora.org>
Change-Id: I0722edbb739140415b49e183be910730f450766c