drm/exynos: use gem create function generically

this patch addes exynos_drm_gem_init() creating and initialzing a gem.
allocation functions could use this function to create new gem and
it changes size type of exynos_drm_gem_create structure to 64bit
and also corrects comments to exynos_drm_gem_create structure.

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