drm/sti: Store correct CRTC index in events

A negative pipe causes a special case to be triggered for drivers that
don't have proper VBLANK support. STi does support VBLANKs, so there is
no need for the fallback code.

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