drm/exynos: cleanup exynos_drm_fbdev_update()

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