drm/exynos: set plane possible_crtcs in exynos_plane_init

All Exynos planes are assigned to exactly one CRTC, it allows to simplify
initialization by moving setting of possible_crtcs to exynos_plane_init.

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