dt-bindings: MAINTAINERS: drop stale exynos file pattern

With last TXT binding converted to DT schema, all Samsung Exynos display
bindings are in "samsung" directory, already present in maintainers
entry.  Drop old "exynos" directory to fix get_maintainers.pl self-test
warning:

  ./MAINTAINERS:7539: warning: no file matches	F:	Documentation/devicetree/bindings/display/exynos/

Fixes: ad6d17e1 ("dt-bindings: display: samsung,exynos5-dp: convert to DT Schema")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>