drm/exynos: dsi: remove global variable exynos_dsi_display

exynos_dsi_display is used by internal Exynos DRM framework for
representing pair encoder->connecter. As it should be mapped 1:1 to dsi
private context it seems more reasonable to embed it directly
in that context. As a result further code simplification will be possible.
Moreover it will be possible to handle multiple DSI devices in the system.

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