drm/panel: s6e3ha2: add VR mode support

drm/panel: s6e3ha2: add VR mode support

This patch adds VR mode support.

For this, it creates a new sysfs file which is used by user-space
to enable or disable VR mode like below,

To enable,
echo 1 > /sys/devices/platform/soc/13900000.dsi/13900000.dsi.0/vr

To disable,
echo 0 > /sys/devices/platform/soc/13900000.dsi/13900000.dsi.0/vr

Actually, this patch enables mDNIe feature of Panel device and
updates its Display color temperature to 6500K for VR mode.

Change-Id: I4e9f15134f57fa200e63ac8fa9d94c5300d6a340
Signed-off-by: Inki Dae <inki.dae@samsung.com>