drm/exynos: remove exynos_encoder's .commit() op

.commit() is not used anymore, Exynos encoders now follow the
.enable()/.disable() semantics from drm atomic core, so remove this
callback.

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