Panels must be initialised with drm_panel_init(). Add the missing
function call in the panel-raspberrypi-touchscreen.c and
panel-sitronix-st7789v.c drivers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190823193245.23876-2-laurent.pinchart@ideasonboard.com