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

The tegra_sor_hdmi_find_settings() function returns NULL on error and
not an ERR_PTR.

Fixes: 459cc2c6 ('drm/tegra: sor: Add HDMI support')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>