dt-bindings: display/msm/hdmi: drop obsolete GPIOs from schema

dt-bindings: display/msm/hdmi: drop obsolete GPIOs from schema

The commit 68e674b13b17 ("drm/msm/hdmi: drop unused GPIO support")
dropped support for obsolete qcom,hdmi-tx-mux-* gpios. They were not
used by any of the upstream platforms. Drop them from the bindings too.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/651708/
Link: https://lore.kernel.org/r/20250505-fd-hdmi-hpd-v5-1-48541f76318c@oss.qualcomm.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>