drm/exynos: remove struct *_win_data abstraction on planes

struct {fimd,mixer,vidi}_win_data was just keeping the same data
as struct exynos_drm_plane thus get ride of it and use exynos_drm_plane
directly.

It changes how planes are created and remove .win_mode_set() callback
that was only filling all *_win_data structs.

v2: check for return of exynos_plane_init()

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>