drm/exynos: fix wrong return check for platform_device_register_simple

platform_device_register_simple() never returns NULL, but IS_ERR_OR_NULL macro
is used for checking return value in exynos drm driver.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>