drm/exynos: Fix potential NULL pointer dereference

drm_mode_create() returns NULL if it fails to create
a new display mode. Check the value returned to avoid NULL
pointer deferencing later.

Change-Id: I2c3747f725eb26becb5436405c93007b85380cdc
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>