drm/exynos: cleanup exynos_user_fb_create()

Using exynos_drm_framebuffer_init(), redundant codes can be removed.

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>