drm/exynos/hdmi: use constant size array for regulators

Driver always uses the same number of regulators, so there is no point in
dynamic allocation.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>