drm/sti: fix potential crash in gdp

In some cases last_close() could be called before sti_gdp_disable()
and make kernel crash because mixer structure has been destroy.
Let's gdp keep a reference on vtg to fix that (like it is already done
in HQVDP)

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Vincent Abriou <vincent.abriou@st.com>