drm/exynos: hdmi: make 'hdmi-en' regulator optional and keep it enabled

HDMI_EN regulator is additional regulator for providing voltage source
for DCC lines available on HDMI connector. When there is no power
provided for DDC epprom, some TV-sets do not pulls up HPD (hot plug
detect) line, what causes HDMI block to stay turned off. This patch
enables HDMI_EN regulator (if available) on driver probe and keep it
enabled all the time to let TV-set correctly signal HPD event.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>