drm/exynos: fixed size type.

size type of drm_exynos_gem_mmap struct is changed to uint64_t and
it adds pad for the struct to be aligned as 64bit.

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