drm/exynos/hdmi: fix pipeline disable order

Hardware require that MIXER(crtc) should be disabled prior to
HDMI(encoder). It was achieved by disabling crtc from encoder disable
callback, bypassing drm core. As a result drm core tried to call vblank
related routines on disabled crtc. The patch fixes it by simplifying
hdmi_disable routine - now it only cancels hotplug worker. Hardware will
be disabled in proper moment during pipe clock disable.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>