drm: Fix kernel doc for DRM_MODE_PROP_IMMUTABLE

This patch explains the DRM_MODE_PROP_IMMUTABLE flag a bit better
by telling which function to call if kernel wants to update
drm object's immutable properties.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181002215055.8652-1-manasi.d.navare@intel.com