drm/mcde: Fix reference to DOC comment

The :doc: reference did not match the DOC comment's name.

Fixes: 5fc537bf

 ("drm/mcde: Add new driver for ST-Ericsson MCDE")
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191002153827.23026-1-j.neuschaefer@gmx.net