egl/wayland: Pull color buffers from dri2_surf->color_buffers pool

egl/wayland: Pull color buffers from dri2_surf->color_buffers pool

We used to keep the color buffers in the dri_buffers array and
swap __DRI_BUFFER_BACK_LEFT and __DRI_BUFFER_FRONT_LEFT around there
and swap third_buffer in in case we needed to triple buffer.  That
gets a little fidgety with all the swaps, so lets use the
color_buffers pool like the gbm platform does.  We track the color buffers,
their corresponding wl_buffer and locked status here and just plug
a free one into dri2_surf->buffers when we need to.

This is a nice clean-up in itself, but it also sets us up to track
buffer age in the color_buffers structs.

Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>