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 <[e-mail]>
Acked-by: Daniel Vetter <[e-mail]>
Acked-by: Dave Airlie <[e-mail]>
Acked-by: Thomas Zimmermann <[e-mail]>
Signed-off-by: Thomas Zimmermann <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/Kconfig

···
 	  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