drm/exynos: make sure to handle an error case to vm_mmap call

vm_mmap function returns unsigned long so addr type should be unsigned long.
a pointer or address variable is required to use unsigned long or uint64_t
type for 64bits address support.

So this patch makes sure that addr has unsigned long type and also
exynos_drm_gem_mmap_ioctl returns correct error type.

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