docs: dt: display/ti: fix typos at the devicetree/ directory name

The name of the devicetree directory is wrong on those three
TI bindings:

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/875b824ac97bd76dfe77b6227ff9b6b2671a6abf.1584450500.git.mchehab+huawei@kernel.org