drm/exynos: fold encoder setup into exynos_drm_load()

As we are removing the exynos encoder move the encoder setup operation
directly inside the exynos_drm_load()

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