drm/exynos/dpi: embed display into private context

exynos_drm_display is used by internal Exynos DRM framework for
representing encoder:connector pair. As it should be mapped 1:1 to dpi
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 dpi devices in the system.

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