MAINTAINERS: dt: update display/allwinner file entry

Changeset f5a98bfe ("dt-bindings: display: Convert Allwinner display pipeline to schemas")
split Documentation/devicetree/bindings/display/sunxi/sun4i-drm.txt
into several files. Yet, it kept the old place at MAINTAINERS.

Update it to point to the new place.

Fixes: f5a98bfe ("dt-bindings: display: Convert Allwinner display pipeline to schemas")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/1be758765272ba4c2acbc3904bdf71c863a90186.1586881715.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>