drm/exynos: checking for NULL instead of IS_ERR()

The of_drm_find_panel() function returns error pointers and never NULL
but we the driver assumes that ->panel is NULL when it's not present.

Fixes: 6afb7721 ("drm/exynos: move connector creation to attach callback")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>