drm/exynos: remove DRM_FORMAT_NV12M from plane module

this patch removes DRM_FORMAT_NV12M from plane module because this format
is same as DRM_FORMAT_NV12. DRM_FORMAT_NV12M will be identified by
mode_cmd->handles and mode_cmd->offsets fields internally.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin.park <kyungmin.park@samsung.com>