drm/exynos: use ERR_PTR instead of NULL in exynos_drm_gem_init

For more correct error information.

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