drm/exynos: fix duplicated mutex lock issue

exynos_drm_crtc_dpms function doesn't need mutex lock
because mutex lock was called by drm framework so this
patch removes mutex lock call from that function to avoid
duplicated mutex locking.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>