drm/exynos: remove exynos_drm_crtc_mode_set_commit()

This was just as extra chain in the call stack. We just rename it to
_set_base() and let it do everything alone.

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