The `opencl_get_plane_format` function was incorrectly determining the
value used to set the image channel order. This resulted in all RGB
pixel formats being set to the `CL_RGBA` pixel format, regardless of
whether or not they actually *were* RGBA.

This patch fixes the issue by using the `offset` and depth of components
rather than the loop index to determine the value of `order`.
Signed-off-by: Jarek Samic <cldfire3@gmail.com>
Signed-off-by: Mark Thompson <sw@jkqxz.net>