drm: print plane state normalized zpos value

When dumping plane state print normalized zpos value as done for
the other plane state fields.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180605135407.20214-2-benjamin.gaignard@linaro.org