drm/sti: fix opencoded use of drm_panel_*

Use the drm_panel_(enable|disable|get_modes) functions.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Vincent Abriou <vincent.abriou@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190804201637.1240-9-sam@ravnborg.org