drm/exynos: cleanup to get gem object for fb

Current codes get first gem object and then again get remain gem
objects. They can be unified to one routine.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>