drm/exynos: exynos_drm_fbdev: Fix incorrect usage of IS_ERR_OR_NULL

exynos_drm_framebuffer_init() does not return NULL. Use IS_ERR instead.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>