drm/exynos: remove struct exynos_drm_encoder layer

struct exynos_drm_encoder was justing wrapping struct drm_encoder, it had
only a drm_encoder member and the internal exynos_drm_encoders ops that
was directly mapped to the drm_encoder helper funcs.

So now exynos DRM uses struct drm_encoder directly, this removes
completely the struct exynos_drm_encoder.

v2: add empty .mode_fixup() and .mode_set() to DSI and DPI to avoid null
pointer.

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