drm/exynos: update fb_info via only one function

This patch moves codes to update fb_info into exynos_drm_fbdev_update(),
so fb_info is updated via only one function.

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>