drm/exynos: update exynos_drm_framebuffer_init() for multiple buffers

This modifies exynos_drm_framebuffer_init() to be possible to support
multiple buffers. Then it can be used by exynos_user_fb_create().

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