[media] s5p-tv: Fix incorrect usage of IS_ERR_OR_NULL in hdmi_drv.c

NULL check on clocks obtained using common clock APIs should not
be done. Use IS_ERR only.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>