drm/exynos: merge exynos_drm_buf.c to exynos_drm_gem.c

The struct exynos_drm_gem_obj can have fields of the struct
exynos_drm_gem_buf then don't need to use exynos_drm_buf.c file.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>