drm: Fix modifiers_property kernel doc

The member is called 'modifiers_property' instead of 'modifiers'. Adjust
the kernel docs to match.

Cc: dri-devel@lists.freedesktop.org
Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Daniel Stone <daniels@collabora.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170824191100.10949-11-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>