drm/exynos: hdmi: fix power order issue

This patch resolves page fault issue of Mixer when disabled.

The SFRs of VP and Mixer are updated by Vertical Sync of Timing
generator which is a part of HDMI so the sequence to disable TV
Subsystem should be as following:
	VP -> Mixer -> HDMI

For this, this patch disables Mixer and VP (if used) prior to
disabling HDMI.

Signed-off-by: Inki Dae <inki.dae@samsung.com>