drm: sti: add panel dependency

The newly added sti driver requires the drm_panel helpers,
and we get a link error if they are not enabled

ERROR: "drm_panel_attach" [drivers/gpu/drm/sti/stidvo.ko] undefined!
ERROR: "of_drm_find_panel" [drivers/gpu/drm/sti/stidvo.ko] undefined!

This adds a 'select' statement as we have for the other drivers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Airlie <airlied@gmail.com>