drm/msm/hdmi: drop the hdmi-mux support

With the last (and only) in-kernel user of hdmi-mux regulator, drop it
from the HDMI driver.

Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/488855/
Link: https://lore.kernel.org/r/20220609122350.3157529-6-dmitry.baryshkov@linaro.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>