drm/meson: dw-hdmi: Use devm_regulator_*get_enable*()

Simplify using the devm_regulator_get_enable_optional(). Also drop the
now unused struct member 'hdmi_supply'.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/df0096b5aea2a18d1540cde379c5abf589ccd7c4.1669799805.git.mazziesaccount@gmail.com