drm/exynos: kill mode_set_nofb callback

All Exynos CRTCs are fully configured by .enable callback. The only users
of mode_set_nofb actually did nothing in their callbacks - they immediately
returned because devices were in suspend state - mode_set_nofb is always
called on disabled device.

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