drm/exynos: Add a new function to get gem buffer size

This patch adds a new function to get gem buffer size. And this
funtion could be used for g2d driver or others can get gem buffer
size to check if the buffer is valid or not.

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