drm: Add comments to Kconfig

Add comments to avoid removing DRM_LEGACY config menu,
because DRM_LEGACY menu could list other legacy drivers.
Signed-off-by: Cai Huoqing <cai.huoqing@linux.dev>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Dave Airlie <airlied@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221203102502.3185-9-cai.huoqing@linux.dev

drivers/gpu/drm/Kconfig

@@ -397,7 +397,7 @@ menuconfig DRM_LEGACY
 	  Unless you have strong reasons to go rogue, say "N".
 
 if DRM_LEGACY
-
+# leave here to list legacy drivers
 endif # DRM_LEGACY
 
 config DRM_EXPORT_FOR_TESTS