drm: Fix kerneldoc for DRM_MODE_PROP_IMMUTABLE

Noticed this while browsing the docs.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180713153444.95466-1-seanpaul@chromium.org