dt-bindings: display: Drop unneeded quotes

Cleanup bindings dropping unneeded quotes. Once all these are fixed,
checking for this can be enabled in yamllint.

Reviewed-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org> #display/msm
Link: https://lore.kernel.org/r/20230320233823.2919475-1-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>