video: meson: hdmi-supply regulator should be optional

Some boards don't have such a regulator, and don't need one to enable
HDMI display. Make it optional, fixing hdmi display for those boards.

Also surround the regulator code with a config check on DM_REGULATOR.

Reported-by: Mohammad Rasim <mohammad.rasim96@gmail.com>
Signed-off-by: Maxime Jourdan <mjourdan@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Mohammad Rasim <mohammad.rasim96@gmail.com>