drm/exynos: consider both case of vflip and hflip.

This patch considers both case of vflip and hflip.
If we want that the contents in buffer to be rotated to 180 degree,
then we can use h,vflip or 180 degree.

Changelog v2:
- added EXYNOS_DRM_FLIP_BOTH enum value to avoid build warnning.

Signed-off-by: Eunchul Kim <chulspro.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>