drm/sti: clarify the skip frame/field message

When a frame or a field is skipped, output a Warning message instead of
an Error message.

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