drm/exynos/dsi: remove mode_set callback

The callback was used only to copy provided mode to context for later
usage. Since the mode is always available from crtc atomic state this code
can be removed.

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