radv/meta: move clear color to using push constants

radv/meta: move clear color to using push constants

The color clear value is uniform and needs only to be emitted from
the frag shader, so just push it down via a push constant,
and remove the vertex buffer completely.

The depth clear value needs to be emitted from the vertex
shader, but is only a single value.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>