It can get exynos_gem object via function argument, so no need to call
exynos_drm_fb_gem_obj() in exynos_drm_fbdev_update.

It also can get struct drm_framebuffer *fb via helper->fb, so can remove
a function argument for it.
Signed-off-by: NJoonyoung Shim <jy0922.shim@samsung.com>
Reviewed-by: NGustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: NInki Dae <inki.dae@samsung.com>