glsl: add field to track if xfb_buffer is an explicit or implicit value

glsl: add field to track if xfb_buffer is an explicit or implicit value

Since any of the xfb_* qualifiers trigger the shader to be in
transform feedback mode we need an extra field to track if
the xfb_buffer on interface members was set explicitly since
xfb_buffer will always have a default value.

Reviewed-by: Dave Airlie <airlied@redhat.com>