drm: Fix plane type uabi breakage

Turns out assuming that only stuff in uabi is uabi is a bit naive, and
we have a bunch of properties for which the enum values are placed in
random headers. A proper fix would be to split out uapi include
headers, but meanwhile sprinkle at least some warning over them.

Fixes: 532b3671 ("drm/doc: Polish for drm_plane.[hc]")
Cc: Archit Taneja <architt@codeaurora.org>
Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1474612525-9488-1-git-send-email-daniel.vetter@ffwll.ch