drm/exynos: fix IS_ERR() vs NULL check in probe

The of_drm_find_bridge() does not return error pointers, it returns
NULL on error.

Fixes: dd8b6803 ("exynos: drm: dsi: Attach in_bridge in MIC driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>