drm/exynos/dsi: refactor panel detection logic

Description of drm_helper_hpd_irq_event clearly states that drivers
supporting hotplug events per connector should use different helper -
drm_kms_helper_hotplug_event. To achieve it following changes have
been performed:
- moved down all DSI ops - they require exynos_dsi_disable function
to be defined earlier,
- simplified exynos_dsi_detect - there is no real detection, it just
returns if panel is attached,
- DSI attach/detach callbacks attaches/detaches DRM panel and sets
connector status and other context fields accordingly, all this is
performed under mutex, as these callbacks are asynchronous.

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