drm/exynos: select CEC_CORE if CEC_NOTIFIER

If the s5p-cec driver is a module and the drm exynos driver is built-in, then
the CEC core will be a module also, causing the CEC notifier to fail (will be
		compiled as empty functions).

To prevent this select CEC_CORE if CEC_NOTIFIER is set to ensure the CEC core
is also built into the kernel.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>