drm/exynos: remove mutex locking in pagefault handler

There is no reason to use mutex locking in pagefault handler.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

diff --cc drivers/gpu/drm/exynos/exynos_drm_gem.c
Simple merge