drm: exynos: hdmi: add support to disable video processor in mixer

This patch adds support for disabling the video processor code based
on the platform type. This is done based on a field in the mixer driver
data which changes with the platform variant.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>