drm/exynos: added missed vm area region mapping type.

with this patch, if the memory region is physically non-continuous
then VM_MIXEDMAP is set to vm->vm_flags otherwise VM_PFNMAP.
we had missed this flag setting.

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