drm/stm: ltdc: check crtc state before enabling LIE

Following investigations of a hardware bug, the LIE interrupt
can occur while the display controller is not activated.
LIE interrupt (vblank) don't have to be set if the CRTC is not
enabled.

Signed-off-by: Yannick Fertre <yannick.fertre@st.com>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1579601650-7055-1-git-send-email-yannick.fertre@st.com