drm/stm: ltdc: use DRM_WARN for fifo & transfer error messages

Use DRM_WARN() instead of DRM_DEBUG_DRIVER() to better
inform the user in case of fifo underruns or
transfer errors.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1553874548-31366-1-git-send-email-yannick.fertre@st.com