gpu: drm: stm: Adopt SPDX identifiers

Add SPDX identifiers to files under stm directory

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Vincent Abriou <vincent.abriou@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171206112947.9569-1-benjamin.gaignard@st.com