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>

drivers/gpu/drm/sti/Kconfig

@@ -5,6 +5,7 @@ config DRM_STI
 	select DRM_KMS_HELPER
 	select DRM_GEM_CMA_HELPER
 	select DRM_KMS_CMA_HELPER
+	select DRM_PANEL
 	select FW_LOADER_USER_HELPER_FALLBACK
 	help
 	  Choose this option to enable DRM on STM stiH41x chipset